RS-232 interface standard

The RS-232C interface standard was issued in the USA in 1969 to define the electrical and mechanical details of the interface between data terminal equipment (DTE) and data communications equipment (DCE) which employ serial binary data interchange.

In serial Data Communications the communications system might consist of:

• The DTE, a data sending terminal such as a computer, which is the source of the data (usually a series of characters coded into a suitable digital form)
• The DCE, which acts as a data converter (such as a modem) to convert the signal into a form suitable for the communications link e.g. analog signals for the telephone system
• The communications link itself, for example, a telephone system
• A suitable receiver, such as a modem, also a DCE, which converts the analog signal back to a form suitable for the receiving terminal
• A data receiving terminal, such as a printer, also a DTE, which receives the digital pulses for decoding back into a series of characters

Figure 1.3 illustrates the signal flows across a simple serial data communications link.

The RS-232C interface standard describes the interface between a terminal (DTE) and a modem (DCE) specifically for the transfer of serial binary digits. It leaves a lot of flexibility to the designers of the hardware and software protocols. With the passage of time, this interface standard has been adapted for use with numerous other types of equipment such as personal computers (PCs), printers, programmable controllers, programmable logic controllers (PLCs), instruments and so on. To recognize these additional applications, the latest version of the standard, RS-232E has expanded the meaning of the acronym DCE from ‘data communications equipment’ to the more general ‘data circuit-terminating equipment”.

RS-232 has a number of inherent weaknesses that make it unsuitable for data communications for instrumentation and control in an industrial environment. Consequently, other RS interface standards have been developed to overcome some of these limitations. The most commonly used among them for instrumentation and control systems are RS-423, RS-422 and RS-485. These will be described in more detail in Chapter 3.

RS-423 interface standard

The RS-423 interface standard is an unbalanced system similar to RS-232 with increased range and data transfer rates and up to 10 line receivers per line driver.

RS-422 interface standard

The RS-422 interface system is a balanced system with the same range as RS-423, with increased data rates and up to 10 line receivers per line driver.

RS-485 interface standard

The RS-485 is a balanced system with the same range as RS-422, but with increased data rates and up to 32 transmitters and receivers possible per line.

The RS-485 interface standard is very useful for instrumentation and control systems where several instruments or controllers may be connected together on the same multi-point network.

Objectives

When you have completed study of this chapter you will be able to:

• Explain the basics of the binary numbering system – bits, bytes and characters
• Describe the factors that affect transmission speed:
  • – Bandwidth
  • – Signal-to-noise ratio
  • – Data throughput
  • – Error rate
• Explain the basic components of a communication system
• Describe the three communication modes
• Describe the message format and error detection in asynchronous communication systems
• List and explain the most common data codes:
  • – Baudot
  • – ASCII
  • – EBCDIC
  • – 4-bit binary code
  • – Gray code
  • – Binary coded decimal (BCD)
• Describe the message format and error detection in synchronous communication systems
• Describe the universal asynchronous transmitter/receiver

2.1 Bits, bytes and characters

A computer uses the binary numbering system, which has only two digits, 0 and 1. Any number can be represented by a string of these digits, known as bits (from binary digit). For example, the decimal number 5 is equal to the binary number 101.

As a bit can have only two values, it can be represented by a voltage that is either on (1) or off (0). This is also known as logical 1 and logical 0. Typical values used in a computer are 0 V for logical 0 and +5 V for logical 1, although it could also be the other way around i.e. 0 V for 1 and +5 V for 0.

A string of eight bits is called a ‘byte’ (or octet), and can have values ranging from 0 (0000 0000) to 255₁₀ (1111 1111₂). Computers generally manipulate data in bytes or multiples of bytes.

Programmers use ‘hexadecimal’ notation because it is a more convenient way of defining and dealing with bytes. In the hexadecimal numbering system, there are 16 digits (0–9 and A–F) each of which is represented by four bits. A byte is therefore represented by two hexadecimal digits.

A ‘character’ is a symbol that can be printed. The alphabet, both upper and lower case, numerals, punctuation marks and symbols such as ‘*’ and ‘&’ are all characters. A computer needs to express these characters in such a way that they can be understood by other computers and devices. The most common code for achieving this is the American Standard Code for Information Interchange (ASCII) described in section 2.8.

2.2 Communication principles

Every data communications system requires:

• A source of data (a transmitter or line driver), which converts the information into a form suitable for transmission over a link
• A receiver that accepts the signal and converts it back into the original data
• A communications link that transports the signals. This can be copper wire, optical fiber, and radio or satellite link

In addition, the transmitter and receiver must be able to understand each other. This requires agreement on a number of factors. The most important are:

• The type of signaling used
• Defining a logical ‘1’ and a logical ‘0’
• The codes that represent the symbols
• Maintaining synchronization between transmitter and receiver
• How the flow of data is controlled, so that the receiver is not swamped
• How to detect and correct transmission errors

The physical factors are referred to as the ‘interface standard’; the other factors comprise the ‘protocols’.

The physical method of transferring data across a communication link varies according to the medium used. The binary values 0 and 1, for example, can be signaled by the presence or absence of a voltage on a copper wire, by a pair of audio tones generated and decoded by a modem in the case of the telephone system, or by the use of modulated light in the case of optical fiber.

2.3 Communication modes

In any communications link connecting two devices, data can be sent in one of three communication modes. These are:

• Simplex
• Half duplex
• Full duplex

A simplex system is one that is designed for sending messages in one direction only. This is illustrated in Figure 2.1.

A duplex system is designed for sending messages in both directions.

Half duplex occurs when data can flow in both directions, but in only one direction at a time (Figure 2.2).

In a full-duplex system, the data can flow in both directions simultaneously (Figure 2.3).

2.4 Asynchronous systems

An asynchronous system is one in which each character or byte is sent within a frame. The receiver does not start detection until it receives the first bit, known as the ‘start bit’. The start bit is in the opposite voltage state to the idle voltage and allows the receiver to synchronize to the transmitter for the following data in the frame.

The receiver reads in the individual bits of the frame as they arrive, seeing either the logic 0 voltage or the logic 1 voltage at the appropriate time. The ‘clock’ rate at each end must be the same so that the receiver looks for each bit at the time the transmitter sends it. However, as the clocks are synchronized at the start of each frame, some variation can be tolerated at lower transmission speeds. The allowable variation decreases as data transmission rates increase, and asynchronous communication can have problems at high speeds (above 100 kbps).

Message format

An asynchronous frame may have the following format:

Start bit:	Signals the start of the frame
Data:	Usually 7 or 8 bits of data, but can be 5 or 6 bits
Parity bit:	Optional error detection bit
Stop bit(s):	Usually 1, 1.5 or 2 bits. A value of 1.5 means that the level is held for 1.5 times as long as for a single bit

An asynchronous frame format is shown in Figure 2.4. The transmitter and receiver must be set to exactly the same configuration so that the data can be correctly extracted from the frame. As each character has its own frame, the actual data transmission speed is less than the bit rate. For example, with a start bit, seven data bits, one parity bit and one stop bit, there are ten bits needed to send seven bits of data. Thus the transmission of useful data is 70% of the overall bit rate.

2.5 Synchronous systems

In synchronous systems, the receiver initially synchronizes to the transmitter’s clock pulses, which are incorporated in the transmitted data stream. This enables the receiver to maintain its synchronization throughout large messages, which could typically be up to 4500 bytes (36 000 bits). This allows large frames to be transmitted efficiently at high data rates. The synchronous system packs many characters together and sends them as a continuous stream, called a packet or a frame.

Message format

A typical synchronous system frame format is shown below in Figure 2.5.

Preamble:	This comprises one or more bytes that allow the receiving unit to synchronize with the frame.
SFD:	The start of frame delimiter signals the beginning of the frame.
Destination:	The address to which the frame is sent.
Source:	The address from which the frame originated.
Length:	The number of bytes in the data field.
Data:	The actual message.
FCS:	The frame check sequence is for error detection.

Each of these is called a field.

2.6 Error detection

All practical data communications channels are subject to noise, particularly copper cables in industrial environments with high electrical noise. Refer to Chapter 6 for a separate discussion on noise. Noise can result in incorrect reception of the data.

The basic principle of error detection is for the transmitter to compute a check character based on the original message content. This is sent to the receiver on the end of the message and the receiver repeats the same calculation on the bits it receives. If the computed check character does not match the one sent, we assume an error has occurred. The various methods of error detection are covered in Chapter 4.

The simplest form of error checking in asynchronous systems is to incorporate a parity bit, which may be even or odd.

Even parity requires the total number of data bits at logic 1 plus the parity bit to equal an even number. The communications hardware at the transmission end calculates the parity required and sets the parity bit to give an even number of logic 1 bits.

Odd parity works in the same way as even parity, except that the parity bit is adjusted so that the total number of logic 1 bits, including the parity bit, equals an odd number.

The hardware at the receiving end determines the total number of logic 1 bits and reports an error if it is not an appropriate even or odd number. The receiver hardware also detects receiver overruns and frame errors.

Statistically, use of a parity bit has only about a 50% chance of detecting an error on a high speed system. This method can detect an odd number of bits in error and will not detect an even number of bits in error. The parity bit is normally omitted if there are more sophisticated error checking schemes in place.

2.7 Transmission characteristics

2.8 Data coding

An agreed standard code allows a receiver to understand the messages sent by a transmitter. The number of bits in the code determines the maximum number of unique characters or symbols that can be represented. The most common codes are described on the following pages.

ASCII code

The most common character set in the western world is the American Standard Code for Information Interchange, or ASCII (see Table 2.3).

This code uses a 7-bit string giving 128 (27) characters, consisting of:

• Upper and lower case letters
• Numerals 0 to 9
• Punctuation marks and symbols
• A set of control codes, consisting of the first 32 characters, which are used by the
• Communications link itself and are not printable

For example: D = ASCII code in binary 1000100.

A communications link setup for 7-bit data strings can only handle hexadecimal values from 00 to 7F. For full hexadecimal data transfer, an 8-bit link is needed, with each packet of data consisting of a byte (two hexadecimal digits) in the range 00 to FF. An 8-bit link is often referred to as ‘transparent’ because it can transmit any value. In such a link, a character can still be interpreted as an ASCII value if required, in which case the eighth bit is ignored.

The full hexadecimal range can be transmitted over a 7-bit link by representing each hexadecimal digit as its ASCII equivalent. Thus the hexadecimal number 8E would be represented as the two ASCII values 38 45 (hexadecimal) (‘8’ ‘E’). The disadvantage of this technique is that the amount of data to be transferred is almost doubled, and extra processing is required at each end.

ASCII control codes can be accessed directly from a PC keyboard by pressing the Control key [Ctrl] together with another key. For example, Control-A (^A) generates the ASCII code start of header (SOH).

The ASCII Code is the most common code used for encoding characters for data communications. It is a 7-bit code and, consequently, there are only 27 = 128 possible combinations of the seven binary digits (bits), ranging from binary 0000000 to 1111111 or hexadecimal 00 to 7F.

Each of these 128 codes is assigned to specific control codes or characters as specified by the following standards:

• ANSI-X3.4
• ISO-646
• ITU alphabet #5

The ASCII Table is the reference table used to record the bit value of every character defined by the code. There are many different forms of the table, but all contain the same basic information according to the standards. Two types are shown here.

Table 2.3 shows the condensed form of the ASCII Table, where all the characters and control codes are presented on one page. This table shows the code for each character in hexadecimal (HEX) and binary digits (BIN) values. Sometimes the decimal (DEC) values are also given in small numbers in each box.

This table works like a matrix, where the MSB (most significant bits – the digits on the left-hand side of the written HEX or BIN codes) are along the top of the table and the LSB (least significant bits – the digits on the right-hand side of the written HEX or BIN codes) are down the left-hand side of the table. Some examples of the HEX and BIN values are given below:

Table 2.4 and Table 2.5 show the form commonly used in printer manuals, sometimes also called the ASCII Code Conversion Table, where each ASCII character or control code is cross referenced to:

• BIN :     A 7-bit binary ASCII code
• DEC :    An equivalent 3 digit decimal value (0 to 127)
• HEX :    An equivalent 2 digit hexadecimal value (00 to 7F)

The UART transmitter

A byte received from the microprocessor for transmission is written to the I/O address of the UART’s transmission sector. The bits to be transmitted are loaded into a shift register, then shifted out on the negative transition of the transmit data clock. This pulse rate sets the baud rate. When all the bits have been shifted out of the transmitter’s shift register, the next packet is loaded and the process is repeated. The word ‘packet’ is used to indicate start, data, parity and stop bits all packaged together. Some authors refer to the packet as a serial data unit (SDU).

Between the transmitter holding register and the shift register is a section called the SDU (serial data unit) formation. This section constructs the actual packet to be loaded into the shift register.

In full duplex communications, the software needs to only test the value of the transmitter buffer empty (TBE) flag to decide whether to write a byte to the UART. In half-duplex communications, the modem must swap between transmitter and receiver states. Hence, the software must check both the transmitter buffer and the transmitter’s shift register, as there may still be some data there.

The UART receiver

The UART receiver continuously monitors the incoming serial line waiting for a start bit. When the start bit is received, the receiver line is monitored at the selected baud rate and the successive bits are placed in the receiver’s shift register. This takes place according to the format described in the user programmable data format register. After assembly of the byte, it is moved into a FIFO (first in first out) buffer. At this stage, the RxRDY (receiver ready) flag is set true and remains true until all the contents of the FIFO buffer are empty.

Typical errors

Another major function of the UART is to detect errors in the data received. Most errors are receiver errors. Typical errors are:

Break detect

To gain the attention of a receiver, a transmitter may hold the data line in a space condition (+voltage) for a period of time longer than that required for a complete character. This is called a break, and receivers can be equipped with a break detect to detect this condition. It is useful for interrupting the receiver, even in the middle of a stream of characters being sent. The break detect time is a function of the baud rate.

Serialization errors are reported in the serialization status register as shown in Figure 2.8 and Figure 2.9.

Receiver timing

It is necessary to have separate clock signals for the UART’s internal operations and to control the shifting operations in the transmitter and receiver sections. The frequency of the master signal is designed to be many times higher than that of the baud rate. This ratio of master serial clock to baud rate is called the clocking factor (typically 16). Instead of sampling the input line at the baud rate frequency, the improved start bit detector samples the incoming line at the rate of the master clock. This minimizes the possibility of an error due to slippage of sampling a stream of serial bits and sampling the wrong bit.

The earliest serial ports used 8250 or 8251 chips, which interrupted the main processor for every character to be transmitted or received. This worked well for the speeds of that time. This has since been replaced by the 16450 chip which works in a similar fashion but supported faster PC bus speeds, and later by 16550 which has a 16-byte buffer thereby reducing the number of CPU interruptions by a factor of 16. A more recent development is to use an enhanced serial port, which provides a buffer of about 1000 bytes and has its own processor to reduce the interruptions to the main CPU by a factor of 1000.

Speed

The 16550 can operate at speeds from 1 to 115 k baud. The 16550 is commonly used on RS-232 even though the RS-232 standard only allows communication at speeds up to 19.2 k baud. Due to the availability and low cost of 16550 chips, the manufacturers of computers and add-on COM ports have included the 16550 as standard equipment.

FIFO buffer

The old 8250 UART (19.2 k) had only a one-byte FIFO buffer. The advantages of the 16-byte buffer on the 16550 are twofold:

• The 16550 makes high speed communications much more reliable.
  On older chips, with their one-byte buffer, the UART would lose data if a second byte came in to the UART before the CPU had a chance to retrieve the first byte. The 16550, with its 16-byte buffer, gives the CPU up to 16 chances to retrieve the data before a character is lost. To realize what this means, if a UART is running at 19 200 bps (with a 10-bit character frame) the CPU will need to service the COM port 1920 times each second or once every .0005 seconds. If the CPU happens to take .0006 seconds to get around to servicing the COM port then the first byte is lost in the one-byte buffer UART. On the 16550 chip, with 16 bytes of buffer space, you can have up to 0.008 seconds to service the COM port.
• It helps make a multitasking system more efficient.
  When the COM port is transmitting data, it has to interrupt the CPU and fill the UART’s transmitter buffer. That means that if the CPU is doing a background directory scan the scan will take longer while the COM port attempts to send data out to the outside world.
  In the one-byte buffer UARTs at 19 200 bps the COM port must interrupt the CPU 1920 times each second just to send data out of the COM port. With the 16550, however, it can put up to 16 bytes into the buffer at a time and therefore interrupt the CPU only 120 times each second. This increases the performance of the CPU to COM port system.

Unbalanced transmission

In an unbalanced system, the signal common reference conductor is simultaneously shared by many signals and other electronic circuitry. Only one wire carries the signal voltage, which is referenced to a signal common wire, sometimes called the signal ground. The transmitted signal is the voltage between the signal conductor and the common reference conductor.

Theoretically, unbalanced transmission should work well if the signal currents are small and the common conductor has very low impedance. In practice, unbalanced systems only work over short communication links. The signal common conductor has characteristics similar to other conductors (resistance, inductance and capacitance) and is not a perfect reference point. For long communication distances, the common conductor does not have the same zero voltage at all points along its length or at its ends. The common conductor can also pick up noise and have other voltages superimposed on it. Sometimes the shield conductor is used as the common reference wire. This practice can introduce excessively high noise-levels and should be avoided. Unbalanced transmission is used in the RS-232 and RS-423 interfaces.

The fact that the common reference conductor may carry superimposed interference voltages means that the voltages V1, V2, and V3 measured at the receiver will be affected (Figure 3.2).

Balanced transmission

Balanced communication interfaces require two conductors to transmit each signal. The voltage at the receiving end is measured as the voltage difference between these two wires. This is known as a balanced or differential system. This eliminates many of the interference problems associated with the common reference wire.

The balanced transmission line permits a higher rate of data transfer over longer distances. The differential method of data transfer is preferable in industrial applications where noise can be a major problem. The disadvantage is that a balanced system requires two conductors for every signal.

The successful transfer of voltage signals across two conductors in the presence of, say noise or voltage drops is based on the assumption that the conductors have similar characteristics and will be affected equally. It does not mean that noise does not exist in the balanced differential system. The voltages on both conductors should rise and fall together, and the differential voltage should remain the same. The voltage between the signal conductor and the common reference conductor is called the common mode voltage (CMV). The CMV is an indication of the induced voltage or noise on the communication link. Ideally, the CMV on the two wires will cancel out completely. However, the greater the CMV, the higher the likelihood of output voltage distortion and damage to the device.

The receiver circuitry of a 2-wire differential system is designed to ignore or reject the CMV, using a technique called common mode rejection (CMR). The effect of noise on the signal is measured as the ratio of the voltage after passing through the receiver to the CMV. The success of the receiver in rejecting the noise is measured as the common mode rejection ratio (CMRR).

Balanced transmission is used in most of the fast interfaces such as RS-422 and RS-485.

The major elements of RS-232

The RS-232 standard consists of three major parts, which define:

• Electrical signal characteristics
• Mechanical characteristics of the interface
• Functional description of the interchange circuits

Electrical signal characteristics

RS-232 defines electrical signal characteristics such as the voltage levels and grounding characteristics of the interchange signals and associated circuitry for an unbalanced system.

The RS-232 transmitter is required to produce voltages in the range:

• Logic 1: –5 V to –25 V
• Logic 0: +5 V to +25 V
• Undefined logic level: +5 V to –5 V

At the RS-232 receiver the following voltage levels are defined:

• Logic 1: –3 V to –25 V
• Logic 0: +3 V to +25 V
• Undefined logic level: –3 V to +3 V

Note: The RS-232 transmitter requires the slightly higher voltage to overcome voltage drop along the line.

The voltage levels associated with a microprocessor are 0 V to +5 V for transistor–transistor Logic (TTL). A line driver is required at the transmitting end to adjust the voltage to the correct level for the communications link. Similarly, at the receiving end a line receiver is required to translate the voltage on the communications link to the correct voltages for interfacing to the microprocessor.

Modern PC power supplies usually have a standard +12 V output that could be used for the line driver.

The control, or ‘handshaking’, lines have the same range of voltages as transmission of logic 0 and logic 1, except that they are of opposite polarity. This means that:

• A control line asserted or made active by the transmitting device has a voltage range of +5 V to +25 V. The receiving device connected to this control line is allowed a voltage range of +3 V to +25 V.
• A control line inhibited or made inactive by the transmitting device has a voltage range of –5 V to –25 V. The receiving device of this control line is allowed a voltage range of –3 V to –25 V.

At the receiving end, a line receiver is necessary in each data and control line to convert the line voltage levels back to the 0 V and +5 V logic levels required by the internal electronics.

The RS-232 standard defines 25 electrical connections. The electrical connections are divided into four groups:

• Data lines
• Control lines
• Timing lines
• Special secondary functions

Data lines are used for the transfer of data. Data flow is designated from the perspective of the DTE interface. The transmit line, on which the DTE transmits and the DCE receives, is associated with pin 2 at the DTE end and pin 2 at the DCE end. The receive line, on which the DTE receives, and the DCE transmits, is associated with pin 3 at the DTE end and pin 3 at the DCE end. Pin 7 is the common return line for the transmit and receive data lines. The allocations are illustrated in Table 3.2.

Control lines are used for interactive device control, which is commonly known as hardware handshaking. They regulate the way in which data flows across the interface. The four most commonly used control lines are:

It is important to remember that with the handshaking lines, the enabled state means a positive voltage and the disabled state means a negative voltage.

Hardware handshaking is the cause of most interfacing problems. Manufacturers sometimes omit control lines from their RS-232 equipment or assign unusual applications to them. Consequently, many applications do not use hardware handshaking but instead use only the three data lines (transmit, receive and signal common ground) with some form of software handshaking. The control of data flow is then part of the application program. Most of the systems encountered in data communications for instrumentation and control use some sort of software-based protocol in preference to hardware handshaking. These protocols are discussed in Chapter 8.

There is a relationship between the allowable speed of data transmission and the length of the cable connecting the two devices on the RS-232 interface. As the speed of data transmission increases, the quality of the signal transition from one voltage level to another, for example from –25 V to +25 V, becomes increasingly dependent on the capacitance and inductance of the cable.

The rate at which voltage can ‘slew’ from one logic level to another depends mainly on the cable capacitance, and the capacitance increases with cable length. The length of the cable is limited by the number of data errors acceptable during transmission. The RS-232 D&E standard specifies the limit of total cable capacitance to be 2500 pF. With typical cable capacitance having improved from around 160 pF/m to only 50 pF/m, the maximum cable length has extended from around 15 meters (50 feet) to about 50 meters (166 feet).

The common data transmission rates used with RS-232 are 110, 300, 600, 1200, 2400, 4800, 9600 and 19 200 bps. Based on field tests, Table 3.1 shows the practical relationship between selected Baud rates and maximum allowable cable length, indicating that much longer cable lengths are possible at lower Baud rates. Note that the achievable speed depends on the transmitter voltages, cable capacitance (as discussed above) as well as the noise environment.

Table 3.1
Demonstrated maximum cable lengths with RS-232 interface

Mechanical characteristics of the interface

RS-232 defines the mechanical characteristics of the interface between the DTE and the DCE. This section dictates that the interface must consist of a plug and socket and that the socket will normally be on the DCE. The familiar DB-25 connector is specified together with a smaller 26 pin alternative connector.

Although not specified by RS-232C, the DB-25 connector (25 pin, D-type) is closely associated with RS-232 and became the de facto standard with revision D. Revision E formally specifies a new connector in the 26-pin alternative connector (known as the ALT A connector). This connector supports all 25 signals associated with RS-232. ALT A is physically smaller than the DB-25 and satisfies the demand for a smaller connector suitable for modern computers. Pin 26 is not currently used. On most RS-232 compatible equipment, where little or no handshaking is required, the DB-9 connector (9 pin, D-type) is common. This practice originated when IBM decided to make a combined serial/parallel adapter for the AT personal computer. A small connector format was needed to allow both interfaces to fit onto the back of a standard ISA interface card. Subsequently, the DB-9 connector has also became an industry standard to reduce the wastage of pins. The pin allocations commonly used with the DB-9 and DB-25 connectors for the RS-232 interface are shown in Table 3.2. The pin allocation for the DB-9 connector is not the same as the DB-25 and often traps the unwary.

The data pins of a DB-9 IBM connector are usually allocated as follows:

• Data transmit pin 3
• Data receive pin 2
• Signal common pin 5

Functional description of the interchange circuits

RS-232 defines the function of the data, timing, and control signals used at the interface of the DTE and DCE. However, very few of the definitions in this section are relevant to applications for data communications for instrumentation and control.

The circuit functions are defined with reference to the DTE as follows:

• Protective ground (Shield)
  The protective ground ensures that the DTE and DCE chassis are at equal potentials. (Remember that this protective ground could cause problems with circulating earth currents.)
• Transmitted data (TxD)
  This line carries serial data from the DTE to the corresponding pin on the DCE. The line is held at a negative voltage during periods of line idle.
• Received data (RXD)
  This line carries serial data from the DCE to the corresponding pin on the DTE.
• Request to send (RTS)
  (RTS) is the request to send hardware control line. This line is placed active (+V) when the DTE requests permission to send data. The DCE then activates (+V) the CTS (clear to send) for hardware data flow control.
• Clear to send (CTS)
  When a half duplex modem is receiving, the DTE keeps RTS inhibited. When it is the DTE’s turn to transmit, it advises the modem by asserting the RTS pin. When the modem asserts the CTS, it informs the DTE that it is now safe to send data.
• DCE ready
  Formerly called data set ready (DSR) – the DTE Ready line is an indication from the DCE to the DTE that the modem is ready.
• Signal ground (Common)
  This is the common return line for all the data transmit and receive signals and all other circuits in the interface. The connection between the two ends is always made.
• Data carrier detect (DCD)
  This is also called the received line signal detector. It is asserted by the modem when it receives a remote carrier and remains asserted for the duration of the link.
• DTE ready (data terminal ready)
  Formerly called data terminal ready (DTR) – DTE ready enables, but does not cause, the modem to switch onto the line. In originate mode, DTE ready must be asserted in order to auto dial. In answer mode, DTE ready must be asserted to auto answer.
• Ring indicator
  This pin is asserted during a ring voltage on the line.
• Data signal rate selector (DSRS)
  When two data rates are possible, the higher is selected by asserting DSRS, however, this line is not used much these days.

The half duplex operation of the RS-232 interface

The following description of one particular operation of the RS-232 interface is based on a half duplex data interchange. The description encompasses the more generally used full duplex operation.

Figure 3.7 shows the operation with the initiating user terminal, DTE, and its associated modem, DCE, on the left of the diagram, the remote computer, and its modem on the right.

The following sequence of steps occurs when a user sends information over a telephone link to a remote modem and computer.

• The initiating user manually dials the number of the remote computer.
• The receiving modem asserts the ring indicator line (RI) in a pulsed ON/OFF fashion reflecting the ringing tone. The remote computer already has its data terminal ready (DTR) line asserted to indicate that it is ready to receive calls. Alternatively, the remote computer may assert the DTR line after a few rings. The remote computer then sets its request to send (RTS) line to ON.
• The receiving modem answers the phone and transmits a carrier signal to the initiating end. It asserts the DCE Ready line after a few seconds.
• The initiating modem asserts the data carrier detect (DCD) line. The initiating terminal asserts its DTR, if it is not already high. The modem responds by asserting its DTE ready line.
• The receiving modem asserts its clear to send (CTS) line, which permits the transfer of data from the remote computer to the initiating side.
• Data is transferred from the receiving DTE (transmitted data) to the receiving modem. The receiving remote computer then transmits a short message to indicate to the originating terminal that it can proceed with the data transfer. The originating modem transmits the data to the originating terminal.
• The receiving terminal sets its request to send (RTS) line to OFF. The receiving modem then sets its clear to send (CTS) line to OFF.
• The receiving modem switches its carrier signal OFF.
• The originating terminal detects that the data carrier detect (DCD) signal has been switched OFF on the originating modem and switches its RTS line to the ON state. The originating modem indicates that transmission can proceed by setting its CTS line to ON.
• Transmission of data proceeds from the originating terminal to the remote computer.
• When the interchange is complete, both carriers are switched OFF and in many cases the DTR is set to OFF. This means that the CTS, RTS, and DCE Ready lines are set to OFF.

Full duplex operation requires that transmission and reception occur simultaneously. In this case, there is no RTS/CTS interaction at either end. The RTS line and CTS line are left ON with a carrier to the remote computer.

Summary of the RS/TIA-232 revisions

A summary of the main differences between RS-232 revisions, C, D, and E are discussed below.

Revision D (RS-232D)

The 25-pin D type connector was formally specified. In revision C, reference was made to the D type connector in the appendices and a disclaimer was included revealing that it was not intended to be part of the standard, however it was treated as the de-facto standard.

The voltage ranges for the control and data signals were extended to a maximum limit of 25 volts from the previously specified 15 volts in revision C.

The 15 meter (50 foot) distance constraint, implicitly imposed to comply with circuit capacitance, was replaced by ‘circuit capacitance shall not exceed 2500 pF’. (Standard RS-232 cable has a capacitance of 50 pF/ft.)

Revision E (RS-232E)

Revision E formally specifies the new 26-pin alternative connector, the ALT A connector. This connector supports all 25 signals associated with RS-232, unlike the 9-pin connector, which has become associated with RS-232 in recent years. Pin 26 is currently not used. The technical changes implemented by RS-232E do not present compatibility problems with equipment confirming to previous versions of RS-232.

This revision brings the RS-232 standard into line with international standards CCITT V.24, V.28, and ISO 2110.

Limitations

In spite of its popularity and extensive use, it should be remembered that the RS-232 interface standard was originally developed for interfacing data terminals to modems. In the context of modern requirements, RS-232 has several weaknesses. Most have arisen as a result of the increased requirements for interfacing other devices such as PCs, digital instrumentation, digital variable speed drives, power system monitors and other peripheral devices in industrial plants.

The main limitations of RS-232 when used for the communications of instrumentation and control data in an industrial environment are:

• The point-to-point restriction is a severe limitation when several ‘smart’ instruments are used
• The distance limitation of 15 meters (50 feet) end to end is too short for most control systems
• The 20 kbps rate is too slow for many applications
• The –3 to –25 V and +3 to +25 V signal levels are not directly compatible with modern standard power supplies.

Consequently, a number of other interface standards have been developed by the RS, which overcome some of these limitations. The RS-422 and RS-485 interface standards are increasingly being used for instrumentation and control systems.

The breakout box

The breakout box is an inexpensive tool that provides most of the information necessary to identify and fix problems on data communications circuits, such as the serial RS-232, RS-422, RS-423, RS-485, etc., interfaces and also on parallel interfaces.

A breakout box is connected into the data cable, to bring out all 25 (or 9, 37, 50, etc.) conductors in the cable to accessible test points. Many versions of this equipment are available on the market, from the ‘homemade’ using a back-to-back pair of male and female DB-25 sockets, to fairly sophisticated test units with built in LEDs, switches and test points.

Breakout boxes usually have a male and a female socket and by using 2 standard serial cables the box can be connected in series with the communication link. The 25 test points can be monitored by LEDs, a simple digital multimeter, an oscilloscope, or a protocol analyzer. In addition, a switch in each line can be opened or closed while trying to identify where the problem is.

The major weakness of the breakout box is that, while one can interrupt any of the data lines, it does not help much with the interpretation of the flow of bits on the data communication lines. A protocol analyzer is required for this purpose.

Null modem

Null modems look like DB-25 ‘through’ connectors and are used when interfacing two devices of the same gender (e.g. DTE–DTE, DCE–DCE) or devices from different manufacturers with different handshaking requirements. A null modem has appropriate internal connections between handshaking pins that ‘trick’ the terminal into believing conditions are correct for passing data. A similar result can be achieved by soldering extra loops inside the DB-25 Plug. Null modems generally cause more problems than they cure and should be used with extreme caution and preferably avoided.

Note that if the null modem connects pin 1 (end-to-end) together, thus providing a combined shield; as in Fig 3.10 – you may end up with a ground loop problem.

Loop back plug

This is a hardware plug, which loops back the transmit data pin to receive data pin and similarly for the hardware handshaking lines. This is another quick way of verifying the operation of the serial interface without connecting to another system.

Protocol analyzer

A protocol analyzer is used to display the actual bits on the data line, as well as the special control codes, such as STX, DLE, LF, CR, etc. The protocol analyzer can be used to monitor the data bits as they are sent down the line and compared with what should be on the line. This helps to confirm that the transmitting terminal is sending the correct data and that the receiving device is receiving it. The protocol analyzer is useful in identifying incorrect setting of baud rate, parity, stop bit, noise or incorrect wiring and connection. It also makes it possible to analyze the format of the message and look for protocol errors.

When the problem has been shown not to be due to the connections, baud rate, bits, or parity, then the content of the message will have to be analyzed for errors or inconsistencies. Protocol analyzers can quickly identify these problems.

Purpose built protocol analyzers are expensive devices and it is often difficult to justify the cost when it is unlikely that the unit will be used very often. Fortunately, software has been developed that enables a normal PC to be used as a protocol analyzer. The use of a PC as a test device for many applications is a growing field, and one way of connecting a PC as a protocol analyzer is shown in Figure 3.11.

Typical RS-232 problems

Below is a list of typical RS-232 problems, which can arise because of inadequate interfacing. These problems could equally apply to two PCs connected to each other or to a PC connected to a printer.

Table 3.4
A list of typical RS-232 problems

Hardware test procedure

• Check cabling connections for proper and complete connections. This should be done off-line (no power to the circuit). It maybe helpful to do a continuity check at this point, while the power is off.
• Check the polarity of the connections between each device on the RS-485 network. They should be A to A and B to B. Usually the black wire is B and the white wire is A, but this can be different for the various systems on the market. In most systems, the red wire of the voltmeter is to go on A and the black lead goes on the B. Check for your system by following the manufacturer’s instructions.
• Check the voltage of the RS-485 system when the system is in an idle state. The idle state on most RS-485 systems is when the transmitters are not sending any data. Most RS-485 systems bias their transmitters to a minus voltage output (a mark or ‘1’) using the voltage divider network indicated in an earlier discussion. However, this is not always the case. Some systems bias the lines with a positive voltage (a space or a ‘0’) via the software while others turn the transmitters off (and have no voltage divider network connected externally) with a resultant zero voltage. Remember the true output of the RS-485 driver chip is usually an open collector output. Biasing keeps the lines from floating.
• Check that the shield (if there is one) is connected to ground at only one point. Ensure that there are no other connections to earth or ground. This is of course more important if the RS-485 devices are far away from each other. It would be wise if the devices were far away (100 meters/300 feet or more), to visually check the shield at each of the locations. The shields of different pairs must be connected.
• Check that the single shield ground is properly connected to an independent, clean earth ground. A clean ground is an earth ground that is as free as possible from noise.
• Check for common mode voltage problems. This is a difficult problem to detect, as it can be intermittent and sporadic. Usual symptoms may include intermittent failures and/or unusual events such as every other unit failing to communicate. The common mode voltage can be measured from each line to ground using a hand-held voltmeter. These voltages should not approach the maximum voltage of the driver chip.
• Ensure that the lines are terminated with the correct resistance terminators (if needed or required). The manufacturer’s requirements must be followed.

Solution

The manufacturer’s recommended termination resistors were added, one at each end.

The reflections disappeared and the system worked correctly.

Note 1: Termination resistors on an RS-485 system are not necessarily required for operation but are preferable to reduce reflections (especially above cable lengths longer than 100 m and baud rates exceeding 40 kbaud).

Note 2: Reducing the baud rate (if possible) is another possible solution.

Note 3: Adding the terminating resistors effectively loads down the RS-485 line drivers. This can cause problems of its own. Do not put a resistor in any location except at the ends of the line. Never install a lower resistance than the manufacturer’s specifications.

Example 2

A two wire master/slave RS-485 system had errors in the responses. The situation was:

• Master dispatched request to slave
• The slave responded with the information. Unfortunately, the master had given up and sent out another request.
• The problem was the delay in the slave responding.
• The master then recorded this as a communication error

Solution

The timeout for the master, before sending out another request to the slave for information, was extended.

Alternative solution

An increase in the baud rate may have improved the situation slightly (minor fix).

Example 3

A two wire master/slave RS-485 system had errors in the responses. The situation was:

• Master dispatched request to slave.
• Slave responded very quickly.
• Master transmitter lines had not gone into the high impedance state.
• Hence, the response from the slave was ‘flattened’ by being transmitted into low impedance.
• The master then recorded this as a communication error.

Solution

The master was put into the high impedance state quicker by disabling the RTS line. Obviously, ensure that this does not worsen the situation by disabling the transmitter before it has completed transmitting the signal.

Alternative solution

Slow down the response from the slave so that the master has a chance to go into the high impedance state.

Physical connection configurations

The devices on the GPIB can be connected in one of two ways:

• A star configuration
• A chain (linear) configuration

A star configuration is one in which each instrument is connected directly to the controller by means of a separate GPIB cable. The connectors are all connected to the same port as the controller, as shown in Figure 3.25. A drawback to this simple arrangement is that all devices on the same bus must be relatively close to the controller because of the length limitation of each cable.

The chain configuration, each device, including the controller, is connected to the next in a chain. The controller does not have to be the first or last device in the chain, but can be linked in anywhere as shown in Figure 3.26. A controller is a controller in the sense that it coordinates the events on the bus. Physically and electrically, it is similar to any other device connected to the GPIB. This configuration is usually the most convenient way to connect equipment.

Although the star and chain configurations are suggested for GPIB, connections can be made in any other way, if the following rules are observed:

• All devices are connected to the bus
• No more than 15 devices, including the controller, may be on the bus with no less than two thirds powered on
• Cable length between any two devices may not exceed 4 meters (13.33 feet) with an average separation of 2 meters (6.66 feet) over the entire bus
• Total cable length may not exceed 20 meters (66.66 feet)

A single device on the GPIB can transfer data to up to 14 other devices. The GPIB uses asynchronous handshaking, so the actual data transfer rate is determined by the devices themselves. Generally, the hardware limits the maximum data rate to about 250 kbps.

Device types

There are many thousands of different types of GPIB-compatible devices available for various applications. There are 4 different groups of devices:

• Talkers
• Listeners
• Talkers/listeners
• Controllers

A talker is a one-way communicating device that can only send data to other devices. It does not receive data. The talker waits for a signal from the controller and then places its data on the bus. Common examples are simple digital voltmeters (DVMs) and some A/D converters.

A listener is a one-way communicating device that can only receive data from another device. It does not send data. It receives data when the controller signals it to read the bus. Common examples are printers, plotters, and recorders.

A talker/listener has the combined characteristics of both talkers and listeners, with the limitation that it is never a talker and a listener at the same time. A common example is a programmable one, which is a listener while its range is being set by the controller, and a talker while it sends the results back to the controller. Most modern digital instruments are talker/listeners as this is the most flexible configuration.

A controller manages and controls everything that happens on the GPIB. It is usually an intelligent or programmable device, such as a PC or a microprocessor controlled device. The controller determines which devices will send and which will receive data and when. To avoid confusion in any GPIB application, there can only be one active controller, called the controller in charge (CIC). There can be several controllers, but to avoid confusion, only one can be active at any time. A controller also has the features of a talker/listener. In some cases, when several PCs are simultaneously connected on a GPIB, one is usually configured as the controller and the others configure as talkers/listeners. The controller needs to be involved in every transfer of data. It needs to address a talker and a listener before the talker can send its message to a listener. After the message is sent, the controller unaddresses both units.

Some GPIB configurations do not require a controller, for example, when only one talker is connected to one or more listener. A controller is necessary when the active or addressed talker or listener must be changed.

Electrical or mechanical characteristics

The GPIB bus is carried inside a shielded 24-wire cable with standard connectors at each end. The standard connector used is the ‘Amphenol’ as shown in Figure 3.27. Adding a new device to the bus is done by connecting a new cable in a star or chain configuration. Screws hold one connector securely to the next. Since the 24-pin connectors are usually stackable, it is easy to connect or disconnect devices to the bus.

The 24 lines in each cable consist of 8 data lines and 8 pairs (16) of control and bus management lines. The data lines are used exclusively to carry data, in a parallel configuration (byte by byte), along the bus. The control and bus management lines are used for various bus management tasks that synchronize the flow of data. When data or commands are sent down the bus, the bus management lines distinguish between the two. Detailed knowledge of how these management lines interact is useful but not necessary to effectively use the GPIB.

In the RS-232, the UART is used to coordinate the ‘housekeeping’ activities associated with the serial interface. The full capacity of the microprocessor can then be directed to other duties. In a similar way, the coordination of the GPIB parallel interface is controlled by a GPIB IC. The most common GPIB ICs used for this purpose are the Texas Instruments TMS9914A and the NEC-7210.

GPIB bus structure

The GPIB consists of 8 data lines (DI01–DI08) and 8-pairs of control lines. Three of the eight control line pairs are the handshaking lines, which coordinate the transfer of data (DAV, NRFD and NDAC). The other five pairs are for bus control and management (ATN, REN, IFC, SRQ and EOI). The 8 ‘ground’ wires provide electronic shielding and prevent bus control signals from interfering with one another or from being influenced by external signals.

The signal lines can be separated into three groups:

The eight data lines DI01 to DI08 carry both data and command messages. All commands and most data use the 7-bit ASCII code, in which case the eighth bit, DI08, is either unused or used for parity.

The GPIB uses binary signals to represent the information that is carried on the lines of the bus. It uses the symbols ‘true’ and ‘false’ to represent the two states of voltages on the lines. The GPIB uses the logic convention called ‘low-true’ or negative logic, where the lower voltage state is ‘true’, and the higher voltage states are ‘false’. Standard TTL voltage levels are used. For example, when DAV is ‘true’, the TTL voltage level is low (±0.8 V), when DAV is false, the TTL level is high (±2.0 V). Despite low being ‘true’, no line can be high (i.e. ‘false’), unless all devices on that line allow it to go high. This is convenient when there are several listeners accepting data. The ‘not data accepted’ (NDAC) line cannot go to the ‘false’ state, indicating data accepted, until the last listener has accepted the data. Consequently, the handshaking process waits for the slowest listener on the bus.

Each device connected to the GPIB has a unique device address and must be designed with enough intelligence to identify whether the data or command sent down the data line is meant for it or for another device. Device addresses are arbitrary and are set by the user, usually on a DPI switch on the back of the device, or by programming the device software. Each connected device is identified in the software of the controllers program. The only limitation in choosing a device address is that it must be an integer number in the range 1 to 30.

GPIB handshaking

Data is transmitted asynchronously on the GPIB parallel interface one byte at a time. The transfer of data is coordinated by the handshake voltage signals on the 3 bus control lines (DAV, NDAC and NRFD), called a three-wire interlocked handshake. Handshaking ensures that a talker will put a data byte on the bus only when all listeners are ready and will keep the data on the bus until it has been read by all listeners. It also ensures that listeners will accept data only when a valid byte is available on the bus.

The talker must wait for the NRFD line to go high (false) before any data can be put onto the bus. The NRFD line is controlled by the listeners. Only when NRFD voltage is high (false) are all listeners ready to receive data. The talker then asserts DAV ‘true’ (voltage low) and when the listeners detect the low level on DAV, they read the byte on the data lines. As each listener accepts the data, it releases NDAC. After the last listener has accepted the data, the NDAC line voltage goes high (false) and this signals the talker that the data has been accepted. Only when the data byte has been accepted by all the listeners can the talker allow DAV voltage to go high (false) and remove its data from the bus. Figure 3.29 illustrates this handshaking sequence.

Interface management lines

The other five lines manage the flow of information across the interface:

• ATN
• REN
• IFC
• SRQ
• EOI

Data transfers

Device addresses are any integer in the range of 1 to 30. There are 2 kinds of addresses for a device:

• Talk address (maximum of 15)
• Listen address (maximum of 15)

If a device sees its talk address on the bus, it knows it will act as a talker and will be required to send data. Conversely, when it sees its listen address on the bus, it will be required to act as a listener and receive data.

Both command/response with polling and interrupt driven protocols are allowed under the IEEE-488 specification.

USB – overall structure

The USB is a master/slave, half-duplex, timed communication bus system designed to connect close peripherals and hubs to an IBM compatible PC. It runs at either 1.5 Mbps (low-speed) or 12 Mbps (high-speed). The PC’s software program (using device drivers) creates packets of information that are going to be sent to devices connected on the USB bus. The USB drivers in the computer allocate a certain time within a frame for the information. The packet is then placed in this 1 ms frame that can contain many packets. One frame might contain information for many devices or it may contain information for only one device. The frame is then sent to the physical layer via the USB drivers, and then on to the bus.

The device receives its part of the packet and if necessary formulates a response. It places this response on the bus. The USB drivers in the PC detect the response on the bus and verify that the frame is correct using a CRC checking method. If the CRC indicates that the frame is correct, the software in the PC accepts the response.

The devices connected to the USB bus can also be powered off the bus cable. Devices can use no more than 500 mA. This works well for small scale DAQ devices, larger DAQ devices usually use external power supplies. Both power and communications are on the same cable and connector.

There are many parts in the USB system that make the communication possible. These include:

• Host hubs
• External hubs
• Type A connector
• Type B connector
• Low-speed cables
• High-speed cables
• USB devices
• Host hub controller hardware and driver
• USB software driver
• Device drivers

Topology

The USB uses a pyramid-shaped topology with everything starting at the host hub. The host hub usually consists of two USB ports on the back of the PC. These ports are basically in parallel with each other.

Each port is a four-pin socket with two pins reserved for power and two for communications. The cables from external hubs or USB devices are plugged into the host hub ports. One or both of the ports can be used. It does not matter which one is used if only one connection is being made. If the external device or hub has a removable cable then a ‘type A or type B’ cable is used to make the connection. The ‘A’ plug goes into the back of the PC (host hub) and the ‘B’ plug goes into the device or external hub. If the external hub or device has an in-built cable then the ‘A’ plug is plugged into the host hub port. The socket on the host hub is keyed so the plug will only go in one way. ‘B’ plugs will not go into ‘A’ sockets and vice-versa.

The limitations on cable lengths are very important for the USB system. All cables, even if they come out of repeater hubs, must be counted in the total length of the cables.

Host hubs

The controller chips for the host hub usually reside on the motherboard inside the PC, although the hub could be a PCB in a PCI slot. The host controller does the parallel to serial and serial to parallel conversion from the PCI bus to the USB connectors. Sometimes a pre-processor is used to improve efficiency of the USB system. This host controller and connector combination is called the root hub or host hub. The host hub’s function is to pass the information to and from the PCI bus to the data lines (+D and –D) on the USB socket. The host controller can control the speed at which the USB operates. It also connects power lines (+5 V and ground) to a USB device via the USB cable. The external USB device may be another USB Hub or a USB type device like a printer

The host hub has complete control over the USB ports. This control would include:

• Initialization and configuration
• Enabling and disabling the ports
• Recognizing the speed of devices
• Recognizing that a device has been connected
• Getting information from the application software
• Creating a packet and then frame
• Sending the information on to the bus
• Waiting and recognizing a response
• Error correction
• Recognizing that a device has been disconnected
• Using the port as a repeater

The connectors (Type A and B)

There are two types of connectors, type A and type B. The reason there are two types is that some devices have built in cables while others have removable cables. If the cables were the same, it would be possible to connect a host hub port to another host hub port. Because of the polarity of the connectors, the +5 V would be connected to ground. To keep this from happening the hub’s output ports use type A connectors and the device input ports are type B. This means that it is impossible to connect one hub port to another hub port. On an external hub, the input to the hub is a type B connector unless the cable on the hub is permanently connected (no connector).

Low-speed cables and high-speed cables

The USB standard states that the USB will run at either 1.5 Mbps (slow-speed) or 12 Mbps (fast speed). The USB must have low-speed cables and high-speed cables. This is due to the impedance difference caused by the different frequencies of data transfer. The low-speed cables use untwisted unshielded cable. The data pair is 28 AWG and the power pair is 20–28 AWG. The low-speed cable is used on devices like keyboards and mice. The maximum distance for low-speed cabling is 3 meters (10 feet). The high-speed cables use twisted shielded cable pairs. The data pair is 28 AWG and the power pair is 20–28 AWG. The maximum propagation delay must be less than 30 ns. The maximum distance for high-speed USB is 5 meters (16 feet).

External hubs

The external hubs are used to increase the amount of devices connected to the system. Usually they have four USB output ports and either one type B input connector or a dedicated cable. This cable has a type A plug. It is usually connected to a host hub, but could be connected to the output socket (type A) of another external hub. Even though the external hub is a repeater, it cannot extend the overall length of the system. This is because of the timing requirements of the USB standard.

The external hub is an intelligent device that can control the communication lines and power lines on its USB ports. It is a bi-directional repeater for information coming from the host hub and from USB devices. It talks to and even acts like an external USB device to the host hub. It plays an integral part in the configuration of devices at start up. There is no physical limit to the number of hubs.

USB devices

The USB system supports every existing peripheral that can be connected to a PC. It also can and has been adapted to devices that are not usually considered peripherals. This would include data acquisition devices such as digital I/O modules and analogue input/output modules. All USB devices must be intelligent devices. Smart devices obviously cost more than ‘dumb’ RS-232 and RS-485 connected devices. With this increased cost, the user gets more functions, ease of use and the ability to connect more devices to the PC. With the old non-USB system, the computer was limited to a few devices. The USB system allows 127 devices to be connected to the PC at the same time.

There are two types of USB device:

• Low-speed, and
• High-speed

The low-speed devices are not only limited in their speed but also in features. These devices include keyboards, mice, and digital joysticks. Since these devices put out small amounts of information, they are polled less frequently and are slower than other devices. When high-speed devices access the USB bus, the low-speed device communication is disabled. Turning off the low-speed device ports at the root or external hubs disables the low-speed devices. The hubs re-enable the low-speed ports after receiving a special preamble packet.

High-speed devices like printers, CD–ROMs, and speakers need the speed of the 12 Mbps bus to transfer the large amount of data required for these devices. All high-speed devices see all traffic on the bus. They are never disabled like the low-speed devices. When a device like a microphone is ‘connected’ to the speakers most of the traffic and therefore packets will be used by the audio system. Other traffic like keyboard and mouse functions will have to wait. The host hub controller driver decides who has to wait and how long.

Host hub controller hardware and driver

The host hub controller hardware and software driver controls all transactions. The host hub controller hardware does the physical connections from the PCI bus to the USB connectors. It enables and initializes the host ports one at a time. It determines the speed and direction of data transfer on both host ports. The host controller in conjunction with the host hub software driver determines the frame contents, prioritization of the devices and how many frames are needed for a particular transfer.

USB software driver

The USB software driver handles the interface between the USB devices, the device drivers, and the host hub driver. When it receives a request from a device driver in the PC to access a certain device, it coordinates the request with other device requests from the application software in the PC. It works with the host hub controller driver to prioritize packets before they are loaded into a frame. The USB software driver gets information from the USB devices during device configuration. It uses this information to tell the host hub controller how to communicate to the device.

Device drivers

For each USB device, a device driver must be loaded into the PC. This device driver is a software interface between the external USB device and the application software, the USB software driver and the host hub controller driver. It has information for the other drivers about that particular device’s needs. This information is used to determine things like the type, speed (although that information can be determined physically by the hub ports), priority, and function of the device, as well as the size of packet needed for the transfer of data.

Communication flow

As mentioned before, the USB system is a master/slave, half duplex, timed communication bus system designed to connect peripherals and external hubs. This means that the peripherals cannot initiate a communication on the USB bus. The master (or host) hub has complete control over the transaction. It initiates all communications with hubs and devices. The USB is timed because all frames are sent within a 1 ms time slot. More than one device can place a packet of information inside that 1 ms frame. The host hub driver, in conjunction with the USB software driver determines the size of the packet and how much time each device is allocated in one frame.

If the applications software wants to send or receive some information from a device, it initiates a transfer via the device driver. Either the manufacturer of the device supplies this device driver or it comes with the operating system. The USB driver software then takes the request and places it in a memory location with other requests from other device drivers. Working together the USB driver, the host hub driver and the host hub controller place the request, data, and packets from the device drivers into a 1 ms wide frame. The host controller then transfers the data serially to the host hub ports. Since all the devices are in parallel on the USB bus, all devices ‘hear’ the information (except low-speed devices, unless it is a low-speed transfer. Low-speed devices are turned off when they are not being polled.) If necessary, the host waits for a response. The remote USB device then responds with an appropriate packet of information. If a device does not see any bus activity for 3 ms, it will go into the suspend mode.

There are four types of IN packets (reading information from a device) and three types of OUT packets (sending information out to a device).

Certain devices like mice and keyboards need to be polled (IN packets), but not too often. The USB software driver knows about these devices and schedules a regular poll for them. Included in the response are three levels of error correction. This type of transfer is very reliable. The peripherals are usually low-speed devices and therefore need a distinct low-speed packet to enable them. This packet is called a preamble packet. The preamble packet is sent out before the poll. The low-speed devices are disabled until they receive this preamble packet. Once they are enabled, they hear the poll and respond. Only one device can be polled at a time and therefore only one device will respond. USB has no provisions for multiple responses from devices.

On the other hand, there are devices that need constant attention but polling is not possible. These would be devices like microphones (IN packets), speakers (OUT packets), and CD-ROMs (both types of packets IN and OUT). The transfer rate is very important to these devices. Obviously, they would use the high-speed transfer rate and they would use a large portion of the frame (up to 90%). The receiving device does not respond to the data transfer. This transfer is a one-way data transfer (simplex). This means that error correction is effectively turned off for these types of transfers.

The physical layer

The physical layer of the Universal Serial Bus is based on a differential +/– 3 V dc communication system. It is in some ways very similar to the RS-485 voltage standard. Unfortunately, it does not have the range of RS-485. This is not because of the type of wire used or because of the USB voltage standard itself, but because of the timing requirements of the USB protocol. In order to fit in everything the peripherals do on a USB bus, it was necessary to put very strict time requirements on the USB.

The USB physical standard has many benefits to the user. It is fast – 12 MHz – is very resistant to noise and is very reliable as long as the cabling rules are followed. With standardized cables and connectors, it is very hard for the user to get things wrong when cabling the USB system.

The story goes that one day Bill Gates was watching his new computer being installed. When he saw the number of wires coming out of the back of the computer, he called the CEO at Intel and said, ‘We have to get rid of this mess of cables and connectors’. And as they say, the rest is history.

Connectors

The plugs and sockets on the USB have two wires for data communication and two wires for power. Using bus-powered devices is optional. The pins on the plug are not the same lengths. The power pins are 7.41 mm long and the communication pins are 6.41 mm long. This means that if a cable is plugged in ‘hot’ the power will be applied to the device before the communications lines. More importantly, it also means that when a cable is unplugged the data communications lines will be disconnected before the power. This reduces the possibility of back EMF voltage damaging the equipment. There are two types of connectors for the USB, type A and type B.

Type A is a flat semi-rectangular keyed connector that is used on the host ports, external hubs, and devices. The type B keyed connector is half-round and smaller than the type A connector is. Note that both type A and B plugs have the USB symbol on the top of the connector. This is for orientation purposes.

The hubs and devices all have female sockets, while the cables have a type A male plug on one end and a type B on the other end. This is because if there were a type A on both ends it would be possible to connect two host hub sockets or external hub sockets. Cables that are not removable from the device or external hub only have a type A plug on one end.

Cables

The cables for the USB are specified as either low- or high-speed cables. Both the low- and high-speed cables can use type A connectors, but only a high-speed device can use type B connectors. Detachable cables are therefore always high-speed cables.

Due to that fact that the impedance of a cable is determined in part by the frequency of the signal, the two speeds need two different cables. External hubs are always high-speed units, but they accept low- and high-speed cables. Low-speed devices like keyboards only connect to other low-speed devices using low-speed cables. The ports on the hub can detect the speed of the device on the other end. If the D+ line is pulled high (+3.0 V dc to +3.6 V dc) then the device is considered high-speed. If the D– line is pulled high then the device is considered low-speed.

The low-speed (1.5 Mbps) cable is an unshielded, untwisted data cable. The communication pair is 28 AWG gauge but due to the lack of shielding and twisting, the overall diameter of the cable is smaller than a high-speed cable. The maximum distance for the low-speed cable is 3 meters (10 feet). This includes all host hub ports to external hub as well as the external hub to device cables. Usually on data communication systems, slower data speeds mean longer distances. In this case, the cable is unprotected against noise and because of the FCC restrictions on 1 to 16 Mbps communication the distance is severely limited.

The high-speed (12 Mbps) cable uses shielded twisted pair 28 AWG gauge wire. The maximum distance for high-speed cables is 5 meters (16 feet). Again, this includes all hub-to-hub and hub-to-device connections. The shield is internally connected to chassis ground at both ends. Usually on data communication systems, the ground is connected at only one end, but because the distances are short, this is not a problem.

Note: It is recommended to measure the chassis to chassis ground difference between both devices before making the connection.

The power pair on both low- and high-speed cables is 20 to 28 AWG gauge. The power pair supplies between 500 and 100 mA to external devices at +5 V dc. Every port on a hub provides this power to the devices if enabled by the hub. All hubs can decide if a port has power applied to the connector. If an external hub is itself powered by the bus then it divides the 500 mA up into 100 mA or so per port.

Signaling

When a device is plugged in to a hub, the port on the hub immediately determines the speed of the device. The port looks at the voltage on the D+ and D– lines. If the D+ line goes positive, the port knows that the device is a high-speed device. If the D– line goes positive, the port knows that the device is a low-speed device.

If both D+ and D– voltages fall below 0.8 V dc for more than 2.5 ms, the hub sees this as the device having been disconnected. If the voltage on either line is raised above 2 V dc for more than 2.5 ms, the port sees this as the device is plugged in.

The idle states for low and high-speed devices are opposite each other. For the low-speed device the idle state is the D+ line is a 0 V and the D– is a positive voltage. The idle state for the high-speed devices is such that the D+ is a positive voltage and the D– is 0 V at idle. In most data communications, a positive voltage indicates a zero (0) condition and a one (1) is minus voltage. In the USB system, it is not possible to say this because it uses an encoding system called NRZI.

The voltages used for the differential balanced signaling are:

• Maximum voltage transmitted +3.6 V dc
• Minimum voltage transmitted +2.8 V dc
• Minimum voltage needed to sense a transition +/– 0.2 V dc
• Typical line voltage as seen from the receiver +/– 3 V dc

The USB uses the non-return to zero inverted (NRZI) encoding scheme. In NRZI a ‘1’ is defined as no change or transition of voltage whereas a “0” is a change or transition of voltage. A string of 0s would cause a clock-like data stream. The USB signaling system uses the transition from one voltage to another to synchronize the receivers. A stream of 1s therefore would mean no transitions. This would cause the receiver to lose synchronization. To overcome this problem the USB system uses a 6 of 7 bit stuffing technique. If six or more 1s are to be transmitted in a row, the transmitter stuffs in a 0 (a transition). If the receiver sees six 1s in a row, it knows that the next transition (zero) is to be ignored.

Power distribution

Devices like keyboards and mice need power to operate. This power is supplied by the USB system through the cables and hubs. External hubs can be either self-powered or powered off the bus. The voltage supplied by a USB hub is +5 V dc. The hubs must be able to supply minimum of 100 mA and maximum of 500 mA through each port. If an external hub with four ports is powered off the bus it divides the 500 mA supplied off the bus between the ports. Four times 100 mA equals 400 mA. This leaves 100 mA to run the hub. It is not possible to connect two bus-powered hubs together unless the devices connected to the last hub are self-powered. If the external hub is self-powered (i.e. mains-powered), it should be able to supply 500 mA to each of the ports.

Data link layer

The data link layer within the USB specification defines the USB as a master/slave, half duplex, timed communication bus system designed to connect close peripherals and external hubs. The hardware and software devices such as the host hub controller hardware and driver, USB software driver and device drivers all contribute to the data link layer of the USB.

With all these devices working together, the data link layer accomplishes the following:

• Collects data off the PCI bus via the device drivers
• Processes the information or data
• Verifies, determines and processes the different transfer types
• Calculates and checks for errors in the packets and frames
• Puts the different packets into 1 ms frames
• Checks for start of frame delimiters
• Sends the packets to the physical layer
• Receives packets from the physical layer

Transfer types

A good place to start when looking at the data link layer of the USB is with the four different transfer types. The wide range of devices that the USB has to deal with requires that there be multiple transfer types. These are:

• Interrupt transfer
• Isochronous transfer
• Control transfers
• Bulk transfers

As stated before, two speeds can be used in the USB system. For the most part the data link layer is the same, but there are some differences. The low-speed devices do not support bulk and isochronous transfers. The reason for this will become apparent in the following transfer descriptions.

The interrupt transfer is used for devices that traditionally used IRQ lines. Devices like keyboards, mice, and DAQ cards use the IRQ lines to tell the computer that they needed service. The USB does not support devices that initiate requests to the computer. To overcome this problem the USB driver initiates a poll of those devices that it knows need periodical attention. This poll must be frequent enough so that data does not get lost, but not too frequent, as not to use up much needed bandwidth. When installed, the device determines its minimum requirements for polling. Devices that need to be polled are rarely polled on every frame. The keyboard is typically polled only every 100th frame.

Isochronous transfer is used when the devices need to be written to or read from at a constant rate. This includes devices like microphones and speakers. The transfer can be done in an asynchronous, synchronous or device specific manner, depending on the device. This constant attention requires that the bulk of the bandwidth of the frame be allocated to one or two devices. If too many of these transfers take place at the same time, data could be lost. This type of transfer is not critical for data quality. There is no error correction and lost data or data that contains errors is ignored. Low-speed devices cannot use isochronous transfer because of the small amounts of data being transferred. It is not possible to move data fast enough using low-speed devices. In an isochronous transfer, the maximum amount of data that can be placed in one packet is 1023 bytes. There is no maximum number of packets that can be sent.

Control transfers are used to transfer specific requests and information to specific devices. This method is used mostly during the configuration and initialization cycles. These transfers are very data critical and require a response or acknowledgment from the device. Full error correction is in force for this type of transfer. All devices use this type of transfer at one time or another. These transfers use very little bandwidth but because the device must respond back to the host hub, the frames are dedicated to this one transfer.

Bulk transfers are used to transfer large blocks of data to devices that are not time dependent but where data quality is important. A typical device that would use the bulk transfer method would be a writeable CD or printer. These devices need large amounts of data but there is no time constraint like there is for a speaker. Whether the data get there in this 10 ms block or the next is not a problem. However, they do need correct data, so this type of transfer includes handshaking and full error correction.

Packets and frames

The USB protocol can and often does use a multi-packeted frame format. The USB frame is made up of up to three parts. One frame equals one transaction.

The three parts of the USB frame are:

• The token packet
• The data packet
• Handshaking

Every frame starts with a token packet. The token packet includes the other smaller packets. These include the synchronization pattern, packet type ID and token packet type.

There are four types of token packets:

• Start of frame packets
• In packets
• Out packets
• Setup packets

The start of frame token packet indicates the start of the packet. This tells the receiver that this is the beginning of the 1 ms frame. The ‘in’ packets transfer data in from the devices to the PC. The ‘out’ packets transfer data out from the PC to the device. The ‘set-up’ packet is used to ask the devices or hubs for startup information. They have information for the devices or hubs.

A special packet is only used on low-speed transfers. It is called the preamble packet. It is a shorter packet than the high-speed frame, only holds up to 64 bytes of data and always uses handshaking. It only has three variations, in packet, out packet and setup packet.

At the end of all packets, except for isochronous frames, there is an error correction packet. On high-speed frames, this is a 16 bit CRC, low-speed devices use a 5 bit CRC because of their smaller packets. If a device or host hub sees an end of frame message, it checks the CRC. If the CRC is correct, it assumes that this is the end of the message. If the CRC is not correct and the timeout limit has not been reached, the receiver waits. If the CRC is not correct and the timeout has been reached, the receiver assumes that the frame is not correct.

Application layer (user layer)

The application layer can be divided into two sub-layers, the operating system, (such as Windows 2000) and the device application software (such as a modem application program).

The application layer of the USB standard is really a user layer, because the USB standard does not define a true application layer. What it does define is a user layer that can be used (by an application programmer) to build an application layer.

The operating system user layer includes:

• Commands
• Software drivers
• Hub configuration
• Bandwidth allocation

Device applications would use:

• Commands
• Device drivers
• Device configuration

Specific user layer information can be found in the universal serial bus specifications at the USB Implementers Forum web page at https://www.usb.org.

Conclusion

Designed as a peripheral connection system for the PC, the USB can be adapted to be used on data acquisition systems. Now that the DAQ industry is developing increasingly intelligent data acquisition and control systems, the USB is easily adaptable to modern DAQ. The devices can be either low- or high-speed devices and very quickly and easily connected to a PC. There are many devices on the market now and it is bound to grow in the future. With the plug and play, system incorporated in USB the user does not have to spend hours or even days configuring the DAQ system. These time savings often offset the extra cost of the devices.

The target speed of USB 2.0 is 480 Mbps, as announced by the USB 2.0 promoter group, consisting of Compaq, Hewlett-Packard, Intel, Lucent, Microsoft, NEC, and Philips. The target speed announcement coincides with the release of the USB 2.0 specification draft to industry developers.

Acknowledgments

Information from the following sources has been included in this section:

• Universal Serial Bus Specification, USB Implementers Forum web page at https://www.usb.org
• Universal Serial Bus System Architecture, MindShare Inc., by Don Anderson
• Intel USB Product Specifications Intel 8×930 and 8×931 USB Peripheral Controllers at:
  https://www.intel.com/design/usb/prodbref/29776501.htm
• Other Web sites of interest:
  https://www.lucent.com/micro/suite/usb.html
  https://www-us.semiconductors.philips.com/usb/

Attenuation

Signal attenuation is the decrease in signal amplitude, which occurs as a signal is propagated through a transmission medium.

A limit needs to be set on the maximum length of cable allowable before one or more amplifiers, or repeaters, must be inserted to restore the signal to its original level. The attenuation of a signal increases for higher frequency components. Devices such as equalizers can be employed to equalize the amount of attenuation across a defined band of frequencies.

Limited bandwidth

Essentially, the larger the bandwidth of the medium the closer the received signal will be to the transmitted one.

The Hartley Law is used to determine the maximum data transfer rate of a transmission line, in the absence of noise:

Max. transfer rate (bps) = 2 B log₂ M

where:

B is the bandwidth in Hertz
M is the number of levels per signaling element

For example:

A modem using Phase QAM, four levels per signaling element and a bandwidth on the public telephone network of 3000 Hz, has a maximum data transfer rate calculated by:

Delay distortion

When transmitting a digital signal, the different frequency components arrive at the receiver with varying delays between them. The received signal is affected by delay distortion. Intersymbol interference occurs when delays become sufficiently large that frequency components from different discrete bits interfere with each other. As the bit rate increases, delay distortion can lead to an increasingly incorrect interpretation of the received signal.

Noise

An important parameter associated with the transmission medium is the concept of signal to noise ratio (S/N ratio). The signal and noise levels will often differ by many orders of magnitude, so it is common to express the S/N ratio in decibels where:

S/N ratio = 10 Log₁₀ S/N dB

where:

S is the signal power in watts
N is the noise power in watts

For example:

an S/N ratio of 1 000 000, is referred to as 60 dB

To calculate the maximum theoretical data rate of a transmission medium we use the Shannon-Hartley Law, which states:

Max data Rate = B log₂ (1 + S/N) bps

where:

B is the bandwidth in Hz

For example:

with an S/N ratio of 100, and a bandwidth of 3000 Hz, the maximum theoretical data rate that can be obtained is given by:

Feedback error control

Feedback error control is where the receiver is able to detect the presence of errors in the message sent by the transmitter. The detected error cannot be corrected but its presence is indicated. This allows the receiver to request a retransmission of the message as defined by a specific protocol. The majority of industrial systems use this approach.

The three most important mechanisms for error detection within feedback error control are:

• Character redundancy: parity check
• Block redundancy: longitudinal parity check, arithmetic checksum
• Cyclic redundancy check (CRC)

Character redundancy checks (parity)

Before transmission of a character, the transmitter uses the agreed mechanism of even or odd parity to calculate the necessary parity bit to append to a character.

For example:

If odd parity has been chosen, then ASCII 0100001 becomes 10100001 to ensure that there are an odd number of 1s in the byte.

For even parity, the above character would be represented as 00100001. At the receiving end, parity for the 7 bit data byte is calculated and compared to the parity bit received. If the two do not agree, an error has occurred.

However: If two of the bits in the character 0100001 had changed the character to 00111001, the parity error reporting scheme would not have indicated an error, when in fact there had been a substantial error.

Parity checking provides only minimal error detection, catching only around 60% of errors on high speed systems.

Parity has been popular because:

• It is low in cost and simple to implement electronically
• It allows a quick check on data accuracy
• It is easy to mentally calculate by the engineer verifying the performance of a system

Although parity has significant weaknesses it is still used where the application is not critical, such as transmitting data to a printer, or communicating between adjacent components in a common electrical system where the noise level is low. Parity is appropriate where the noise burst length is expected to not exceed one bit, i.e. only single bit errors can be expected. This means it is only effective for slow systems. Parity error detection is not used much today for communication between different computer and control systems. Sophisticated algorithms, such as block redundancy, longitudinal parity check, and cyclic redundancy check (CRC), are preferred where the application is more critical.

Block redundancy checks

The parity check on individual characters can be supplemented by a parity check on a block of characters. There are two block check methods:

Cyclic redundancy check (CRC)

For longer messages, an alternative approach has to be used. For example, an Ethernet frame has up to 1500 bytes or 12 000 bits in the message. A popular and very effective error checking mechanism is cyclic redundancy checking. The CRC is based upon a branch of mathematics called algebra theory, and is relatively simple to implement. Using a 16 bit check value, CRC promises detection of errors as shown in Table 4.3. (1)

The CRC error detection mechanism is obviously very effective at detecting errors, particularly difficult to handle ‘burst errors’, where an external noise source temporarily swamps the signal, corrupting an entire string of bits. The CRC is effective for messages of any length.

Forward error correction

Forward error correction is where the receiver can not only detect the presence of errors in a message, but also reconstruct the message into what it believes to be the correct form. It may be used where there are long delays in requesting retransmission of messages or where the originating transmitter has difficulty in retransmitting the message when the receiver discovers an error. Forward error correction is generally used in applications such as NASA space probes operating over long distances in space where the turn around time is too great to allow a retransmission of the message.

Hamming codes and hamming distance

In the late 1940s, Richard Hamming and Marcel Golay did pioneering work on error detecting and error correcting codes. They showed how to construct codes which were guaranteed to correct certain specified numbers of errors, by elegant, economic and sometimes optimal means.

Coding the data simply refers to adding redundant bits in order to create a codeword. The extra information in the codeword, allows the receiver to reconstruct the original data in the event of one or more bits being corrupted during transmission.

An effective method of forward error correction is the use of the Hamming codes. These codes detect and correct multiple bits in coded data. A key concept with these codes is that of the Hamming distance. For a binary code, this is just the number of bit positions at which two codewords vary. For instance, the Hamming distance between 0000 and 1001 is 2.

A good choice of code means that the codewords will be sufficiently spaced, in terms of the Hamming distance, to allow the original signal to be decoded even if some of the encoded message is transmitted incorrectly.

The following examples illustrate a Hamming code.

A code with a Hamming distance of 1 could represent the eight alphanumeric symbols in binary as follows:

If there is a change in 1 bit in the above codes, due to electrical noise for example, the receiver will read in a different character and has no way of detecting an error in the character. Consequently, the Hamming distance is 1, and the code has no error detection capabilities.

If the same three bit code is used to represent four characters, with the remaining bit combinations unused and therefore redundant, the following coding scheme could be devised.

This code has a Hamming distance of 2, as two bits at least have to be in error before the receiver reads an erroneous character.

It can be demonstrated that a Hamming distance of three requires three additional bits, if there are four information bits. This is referred to as a Hamming (7,4) code. For a 4-bit information code, a 7-bit code word is constructed in the following sequence:

C₁C₂I₃C₄I₅I₆I₇

where:

I₃I₅I₆I₇ are the information, or useful bits
C₁C₂C₄ are the redundant bits calculated as follows:

– C₁ = I₃ XOR I₅ XOR I₇
– C₂ = I₃ XOR I₆ XOR I₇
– C₄ = I₅ XOR I₆ XOR I₇

For example:

If the information bits are 1101

(I₃ = 1; I₅ = 1; I₆ = 0; I₇ = 1), the Hamming (7,4) codeword is:

– C₁ = 1 XOR 1 XOR 1 = 1
– C₂ = 1 XOR 0 XOR 1 = 0
– C₄ = 1 XOR 0 XOR 1 = 0

The codeword (C₁C₂I₃C₄I₅I₆I₇) is then represented as 1010101.

If one bit is in error and the codeword 1010111 was received, the redundant bits would be calculated as:

– C₁ = 1 XOR 1 XOR 1 = 1
(and matches the 1 from the received codeword)

– C₂ = 1 XOR 1 XOR 1 = 1
(but does not match the 0 from the received codeword)

– C₄ = 1 XOR 1 XOR 1 = 1
(but does not match the 0 from the received codeword)

C₂ and C₄ indicate one bit out of place, which would be either I6 or I7 (as this is common to both). However C1 matches the check calculation, therefore I6 must be in error.

Hence, the code word should be: 1010101.

Theory of operation

The optical fiber forms a wave guide for light with the light being guided through the core of the fiber.

Communication over fiber-optic cables works on the principle that light propagates through different media at different speeds (in the same manner as radio waves). When light moves from one medium of a certain density to another of a different density, the light will change direction. This phenomenon is known as refraction.

The effectiveness of a medium to propagate light can be expressed as a ratio of an absolute reference; light traveling through a vacuum (3 ×10⁸ m/s) i.e. speed of light in free space. This ratio is known as the ‘Refractive Index’ and is calculated as:

In a typical fiber-optic medium, light travels at approximately 2 × 10⁸ m/s. Therefore the refractive index is:

n1 = 1.5

The optical medium is said to have a refractive index of 1.5.

Fiber-optics follows Snell’s law, which states that the ratio of the sine of the angle of incidence (Q_i) to the sine of the angle of refraction (Q_r) is equal to the ratio of the speed of light in the two respective media (C₁/C₂). This is equal to a constant (K), which has a ratio of the refractive index of medium 2 to medium 1 (n₂/n₁).

The formula is:

Fiber-optic cables are manufactured with a pure optical glass core, surrounded by a glass cladding. The core and cladding are treated with an impurity so that their refractive indices are different. Figure 5.2 and Figure 5.3 show the basic construction of the optical fiber. This construction allows the core to guide the light pulses to the receiver.

Because the refractive index of the core and cladding is different, light entering the core at an acceptable angle of entry will propagate the length of the fiber without losing light through the cladding. Light must enter the fiber within a ‘cone of acceptance’ angle. When light attempts to enter at an angle greater than the ‘cone of acceptance’, it will not reflect from the cladding and is lost.

The optical fiber acts as a conduit (or wave guide) for pulses of light generated by a light source. The light source is typically a laser diode or light emitting diode (LED) operating at wavelengths of 0.85, 1.3 or 1.55 micrometers.

The optical fiber is coated with a protective colored sheath to provide environmental protection and easy identification.

Modes of propagation

Fiber types are generally identified by the number of paths that the light follows inside the fiber core called ‘modes’ of propagation. There are two main modes of light propagation through an optic fiber, which give rise to two main constructions of fiber, ‘multimode’, and ‘monomode’ (also known as ‘single mode’).

Multimode fibers are easier and cheaper to manufacture than monomode fibers. Multimode cores are typically 50 times greater than the wavelength of the light signal they will propagate. With this type of fiber, an LED transmitter light source is normally used because it can be coupled with less precision than a laser diode.

With the wide aperture and LED transmitter, the multimode fiber will send light in multiple paths (modes) toward the receiver as illustrated in Figure 5.4.

The light takes many paths between the two ends as it reflects from the sides of the fiber core. This causes the light paths to arrive both out of phase and at different times resulting in a spreading of the original pulse shape. As a result, the original sharp pulses sent from one end become distorted by the time they reach the receiving end.

The problem becomes worse as data rates increase. Multimode fibers, therefore, have a limited maximum data rate (bandwidth) as the receiver can only differentiate between the pulsed signals at a low data rate. The effect is known as ‘modal dispersion’ and its result referred to as ‘intersymbol interference’. For slower data rates over short distances, multimode fibers are quite adequate and speeds of up to 300 Mbps are readily available.

A further consideration, with multimode fibers, is the ‘index’ of the fiber (how the impurities are applied in the core). The cable can be either ‘graded index’ (more expensive but better performance) or ‘step index’ (less expensive), refer to Figure 5.5. The type of index affects the way in which the light waves reflect or refract off the walls of the fiber. Graded index cores focus the modes as they arrive at the receiver, and consequently improve the permissible data rate of the fiber.

The core diameters of multimode fibers typically range between 50–100 µm. The two most common core diameters are 50 and 62.5 µm.

‘Monomode’ or ‘single mode’ fibers are less expensive but more difficult to interface. They allow only a single path or mode for the light to travel down the fiber with minimal reflections. Monomode fibers typically use lasers as light sources.

Monomode fibers do not suffer from major dispersion or overlap problems and permit a very high rate of data transfer over much longer distances. The core of the fibers is much thinner than multimode fibers at approximately 5–10 µm. The cladding diameter is 125 µm, the same as for multimode fibers.

Source lighting must be powerful and aimed precisely into the fiber to overcome any misalignment (hence the use of laser diodes). The thin monomode fibers are difficult to work with when splicing, terminating, and are consequently expensive to install.

A typical application for a monomode installation would be a high capacity telephone link where the traffic volume makes a large bandwidth necessary.

Specification of cables

Optical fibers are specified based on diameter. A fiber specified as 50/150 has a core of 50 µm and a cladding diameter of 150 µm. The most popular sizes of multimode fibers are 50/125, used mainly in Europe, and 62.5/125, used mainly in Australia and the USA.

Another outer layer provides an external protection against abrasion and shock. Outer coatings can range from 250–900 µm in diameter, and very often cable specifications include this diameter, for example: 50/150/250.

To provide additional mechanical protection, the fiber is often placed inside a loose, but stiffer, outer jacket which adds thickness and weight to the cable. Cables made with several fibers are most commonly used. The final sheath and protective coating on the outside of the cable depends on the application and where the cable will be used. A strengthening member is normally placed down the center of the cable to give it longitudinal strength. This allows the cable to be pulled through a conduit or hung from a power pole without causing damage to the fibers. The tensile members are made from steel or Kevlar, the latter being more common. In industrial and mining applications, fiber cores are often placed inside cables used for other purposes, such as trailing power cables for large mining, stacking, or reclaiming equipment.

Experience has shown that optic fibers will break two or three times in a 25 year period. In general, the incremental cost of extra fiber cores in cables is not very high when compared to overall costs (including installation and termination costs). Therefore, it is often worthwhile specifying extra cores as spares, or for future use.

Joining cables

In the early days of optic fibers, connections and terminations were a major problem. Largely, this has improved but connections still require a great deal of care to avoid signal losses that will affect the overall performance of the communications system.

There are three main methods of splicing optic fibers:

To overcome the difficulties of termination, fiber-optic cables can be provided by a supplier in standard lengths such as 10 m, 100 m or 1000 m with the ends cut and finished with a mechanical termination ferrule that allows the end of the cable to slip into a closely matching female socket. This enables the optical fiber to be connected and disconnected as required. The mechanical design of the connector forces the fiber into a very accurate alignment with the socket and results in a relatively low loss. Similar connectors can be used for in-line splicing using a double-sided female connector.

Although the loss through this type of connector can be an order of magnitude greater than the loss of a fused splice, it is much quicker and requires no special tools or training. Unfortunately, mechanical damage or an unplanned break in a fiber requires special tools and training to repair and re-splice. One way around this problem is to keep spare standard lengths of pre-terminated fibers that can quickly and easily be plugged into the damaged section. The techniques for terminating fiber-optic cables are constantly being improved to simplify these activities.

Limitations of cables

On the negative side, the limitations of fiber-optic cables are as follows:

• The cost of source and receiving equipment is relatively high.
• It is difficult to ‘switch’ or ‘tee-off’ a fiber-optic cable so fiber-optic systems are most suitable for point-to-point communication links.
• Techniques for joining or terminating fibers (mechanical and chemical) are difficult and require precise physical alignment. Special equipment and specialized training are required.
• Equipment for testing fiber-optic cables is different and more expensive from traditional methods used for electronic signals.
• Fiber-optic systems are used almost exclusively for binary digital signals and are not suitable for long distance analog signals.

Objectives

When you have completed studying this chapter you will be able to:

• Define the terms noise and SNR
• Explain the frequency spectrum analysis of the three noise groups
• Give examples of electrical noise sources
• Explain the four forms of electrical coupling of noise:
  • – Impedance coupling
  • – Electrostatic coupling
  • – Magnetic/inductive coupling
  • – Electromagnetic radiation
• Explain shielding techniques and shielding performance ratios, including cable ducting and cable spacing
• Describe earthing and grounding requirements
• Describe suppression techniques
• Describe the filtering of specific noise sources

6.1 Definition of noise

Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. In many cases, the noise will be unpredictable due to transients (or spikes) caused, for example, by lightning. In other cases, it may be due to the predictable 50 or 60 Hz ac ‘hum’ from power circuits close to the data communications cable. This unpredictability makes the design of a data communications system quite challenging.

Noise can be generated from within the system itself (internal noise) or from an outside source (external noise).

Examples of noise sources are:

• Internal noise
• Thermal noise (due to electron movement within the electrical circuits)
• Imperfections (in the electrical design)
• External noise
• Natural origins (electrostatic interference and electrical storms)
• Electromagnetic Interference (EMI) – from currents in cables
• Radio Frequency Interference (RFI) – from radio systems radiating signals
• Crosstalk (from other cables separated by a small distance)

It is commonly accepted that the main techniques used to reduce noise consist of

• Applying shielding around the signal wires
• Increasing the distance between the noise source and the signal
• Proper grounding of the shield and twisting of the signal wires

Noise is only important if it is measured in relation to the communication signal, which carries the data information. In previous chapters, it has been demonstrated that electronic receiving circuits for digital communications have a broad voltage range, which determines whether a signal is a binary bit ‘1’ or ‘0’. The noise voltage has to be high enough to take the signal voltage outside these limits for errors to occur.

The ratio of the signal voltage to the noise voltage determines the strength of the signal in relation to the noise. This is called ‘signal to noise ratio’ (SNR) and is important in assessing how well the communication system will operate. In data communications, the signal voltage is relatively stable and is determined by the voltage at the source (transmitter) and the volt drop along the line due to the cable resistance (size and length). SNR is therefore a measure of the interference on the communication link.

The SNR is usually expressed in decibels (dB), which is the logarithmic ratio of the signal voltage (S) to noise voltage (N).

SNR = 10log S/N dB

An SNR of 20 dB is considered low (bad), while an SNR of 60 dB is considered high (good). The higher the SNR, the easier it is to provide acceptable performance with simpler circuitry and cheaper cabling.

In data communications, a more relevant performance measurement of the link is the bit error rate (BER). This is a measure of the number of successful bits received compared to bits that are in error. A BER of 10^–6 means that one bit in a million will be in error and is considered poor performance on a bulk data communications system with high data rates. A BER of 10^–12 (one error bit in a million, million) is considered very good. Over industrial systems, with low data requirements, a BER of 10^–4 could be quite acceptable.

There is a relationship between SNR and BER. As the SNR increases, the error rate drops off rapidly as is shown in Figure 6.1. Most communications systems start to provide reasonably good BERs when the SNR is above 20 dB.

6.2 Frequency analysis of noise

Another useful way of evaluating the effects of noise is to examine its frequency spectrum. Noise can be seen to fall into three groups:

• Wideband noise
• Impulse noise
• Frequency specific noise

The three groups are shown in the simplified frequency domain as well as the conventional time domain. In this way, we can appreciate the signal’s changing properties as well as viewing the amplitude in the customary time domain.

Wideband noise contains numerous frequency components and amplitude values. These are depicted in the time domain graph shown in Figure 6.2 and in the frequency domain graph shown in Figure 6.3.

In the frequency domain, the energy components of wideband noise extend over a wide range of frequencies (frequency spectrum).

Wideband noise will often result in the occasional loss or corruption of a data bit. This occurs at times when the noise signal amplitude is large enough to confuse the system into making a wrong decision on what digital information or character was received. Encoding techniques such as parity checking and block character checking (BCC) are important for wideband error detection so that the receiver can determine when an error has occurred.

Impulse noise is best described as a burst of noise which may last for duration of say up to 20 ms. It appears in the time domain as indicated in Figure 6.4.

The frequency domain illustrates this type of noise. It affects a wide bandwidth with decreasing amplitude versus frequency.

Impulse noise is brought about by the transient disturbances in electrical activity such as when an electric motor starts up, or from switching elements within telephone exchanges. Impulse noise swamps the desired signal, thus corrupting a string of data bits. As a result of this effect, synchronization may be lost or the character framing may be disrupted. Noise of this nature usually results in garbled data making messages difficult to decipher. Cyclic redundancy checking (CRC) error detection techniques may be required to detect such corruption.

Although more damaging than wideband noise, impulse noise is generally less frequent. The time and frequency domain plots for impulse noise will vary depending on the actual shape of the pulse. Pulse shapes may be squared, trapezoidal, triangular or sine for example.

In general, the narrower and steeper a pulse, the more energy is placed in the higher frequency regions.

Frequency specific noise is characterized by a constant frequency, but its amplitude may vary depending on how far the communication system is from the noise source, the amplitude of the noise signal and the shielding techniques used.

This noise group is typical of ac power systems and can be reduced by separating the data communication system from the power source. Because this form of noise has a predictable frequency spectrum, noise resistance is easier to implement within the system design.

Filters are typically used to reduce this to an acceptable level.

6.3 Sources of electrical noise

Typical sources of noise are devices, which produce quick changes (or spikes) in voltage or current, such as:

• Large electrical motors being switched on
• Fluorescent lighting tubes
• Lightning strikes
• High voltage surges due to electrical faults
• Welding equipment

From a general point of view, there must be three contributing factors before an electrical noise problem can exist. These are:

• A source of electrical noise
• A mechanism coupling the source to the affected circuit
• A circuit conveying the sensitive communication signals

6.4 Electrical coupling of noise

There are four forms of coupling of electrical noise into the sensitive data communications circuits. These are:

• Impedance coupling (sometimes referred to as conductance coupling)
• Electrostatic coupling
• Magnetic or inductive coupling
• Radio frequency radiation (a combination of electrostatic and magnetic)

Each of these noise forms will be discussed in some detail in the following sections. Although the order of discussion is indicative of the frequency of problems, this will obviously depend on the specific application.

Impedance coupling (or common impedance coupling)

For situations where two or more electrical circuits share common conductors, there can be some coupling between the different circuits with deleterious effects on the connected circuits. Essentially, this means that the signal current from the one circuit proceeds back along the common conductor resulting in an error voltage along the return bus, which affects all the other signals. The error voltage is due to the capacitance, inductance, and resistance in the return wire. This situation is shown in the Figure 6.8.

Obviously, the quickest way to reduce the effects of impedance coupling is to minimize the impedance of the return wire. The best solution is to use a balanced circuit with separate returns for each individual signal.

Electrostatic or capacitive coupling

This form of coupling is proportional to the capacitance between the noise source and the signal wires. The magnitude of the interference depends on the rate of change of the noise voltage and the capacitance between the noise circuit and the signal circuit.

In the Figure 6.10, the noise voltage is coupled into the communication signal wires through the two capacitors C₁ and C₂, and a noise voltage is produced across the resistances in the circuit. The size of the noise (or error) voltage in the signal wires is proportional to the:

• Inverse of the distance of noise voltage from each of the signal wires
• Length (and hence impedance) of the signal wires into which the noise is induced
• Amplitude (or strength) of the noise voltage
• Frequency of the noise voltage

There are four methods for reducing noise induced by electrostatic coupling. They are:

• Shielding of the signal wires
• Separating from the source of the noise
• Reducing the amplitude of the noise voltage (and possibly the frequency)
• Twisting of the signal wires

Figure 6.11 indicates the situation that occurs when an electrostatic shield is installed around the signal wires. The currents generated by the noise voltages prefer to flow down the lower impedance path of the shield rather than the signal wires. If one of the signal wires and the shield are tied to the earth at one point, which ensures that the shield and the signal wires are at an identical potential, then reduced signal current flows between the signal wires and the shield.

Note: The shield must be of a low resistance material such as aluminum or copper. For a loosely braided copper shield (85% braid coverage), the screening factor is about 100 times or 20 dB i.e. C₃ and C₄ are about 1/100 C₁ or C₂. For a low resistance multi-layered screen, this screening factor can be 35 dB or 3000 times.

Twisting of the signal wires provides a slight improvement in the induced noise voltage by ensuring that C1 and C2 are closer together in value; thus ensuring that any noise voltages induced in the signal wires tend to cancel one another out.

Note: Provision of a shield by a cable manufacturer ensures that the capacitance between the shield and the wires are equal in value (thus eliminating any noise voltages by cancellation).

Magnetic or inductive coupling

This depends on the rate of change of the noise current and the mutual inductance between the noise system and the signal wires. Expressed slightly differently, the degree of noise induced by magnetic coupling will depend on the:

• Magnitude of the noise current
• Frequency of the noise current
• Area enclosed by the signal wires (through which the noise current magnetic flux cuts)
• Inverse of the distance from the disturbing noise source to the signal wires

The effect of magnetic coupling is shown in Figure 6.12.

The easiest way of reducing the noise voltage caused by magnetic coupling is to twist the signal conductors. This results in lower noise due to the smaller area for each loop. This means less magnetic flux to cut through the loop and consequently a lower induced noise voltage. In addition, the noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop. Hence, an even number of loops will tend to have the noise voltages canceling each other out. It is assumed that the noise voltage is induced in equal magnitudes in each signal wire due to the twisting of the wires giving a similar separation distance from the noise voltage. See Figure 6.13.

The second approach is to use a magnetic shield around the signal wires. The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield. These eddy currents then create an opposing magnetic flux ⊘₁ to the original flux ⊘₂. This means a lesser flux (⊘₂ – ⊘₁) reaches our circuit!

Note: The magnetic shield does not require earthing. It works merely by being present. High permeability steel makes best magnetic shields for special applications. However, galvanized steel conduit makes a quite effective shield.

Radio frequency radiation

The noise voltages induced by electrostatic and inductive coupling (discussed above) are manifestations of the near field effect, which is electromagnetic radiation close to the source of the noise. This sort of interference is often difficult to eliminate and requires close attention of grounding of the adjacent electrical circuit and the earth connection is only effective for circuits in close proximity to the electromagnetic radiation. The effects of electromagnetic radiation can be neglected unless the field strength exceeds 1 volt/meter. This can be calculated by the formula:

where:

• – Field strength is in volt/meter
• – Power is in kilowatt
• – Distance is in km

The two most commonly used mechanisms to minimize electromagnetic radiation are:

• Proper shielding (iron)
• Capacitors to shunt the noise voltages to earth

Any incompletely shielded conductors will perform as a receiving aerial for the radio signal and hence care should be taken to ensure good shielding of any exposed wiring.

6.5 Shielding

It is important that electrostatic shielding is only earthed at one point. More than one earth point will cause circulating currents. The shield should be insulated to prevent inadvertent contact with multiple points, which behave as earth points resulting in circulating currents. The shield should never be left floating because this would tend to allow capacitive coupling, rendering the shield useless.

Two useful techniques for isolating one circuit from another are by the use of opto-isolation as shown in the Figure 6.15, and transformer coupling as shown in Figure 6.16.

Although opto-isolation does isolate one circuit from another, it does not prevent noise or interference being transmitted from one circuit to another.

Transformer coupling can be preferable to optical isolation when there are very high speed transients in the one circuit. There is some capacitive coupling between the LED and the base of the transistor, which in the opto-coupler can allow these types of transients to penetrate one circuit from another. This is not the case with transformer coupling.

6.6 Good shielding performance ratios

The use of some form of low resistance material covering the signal conductors is considered good shielding practice for reducing electrostatic coupling. When comparing shielding with no protection, this reduction can vary from copper braid (85% coverage) which returns a noise reduction ratio of 100:1 to aluminum Mylar tape, with drain wire, with a ratio of 6000:1.

Twisting the wires to reduce inductive coupling reduces the noise (in comparison to no twisting) by ratios varying from 14:1 (for four inch lay) to 141:1 (for one inch lay). In comparison, putting parallel (untwisted) wires into steel conduit only gives a noise reduction of 22:1.

On very sensitive circuits with high levels of magnetic and electrostatic coupling the approach is to use coaxial cables. Double shielded cable can give good results for very sensitive circuits.

Note: With double shielding, the outer shield could be earthed at multiple points to minimize radio frequency circulating loops. This distance should be set at intervals of less than 1/8th the wavelength of the radio frequency noise.

6.7 Cable ducting or raceways

These are useful in providing a level of attenuation of electric and magnetic fields. These figures are 60 Hz for magnetic fields and 100 kHz for electric fields.

Typical screening factors are:

• 5 cm (0.2 inch) aluminum conduit with 0.154 inch thickness
  • – Magnetic fields 1.5:1
  • – Electric fields 8000:1
• Galvanized Steel conduit 5 cm (0.2 inch), wall thickness 0.154 inch with
  • – Magnetic fields 40:1
  • – Electric fields 2000:1

6.8 Cable spacing

In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful set of tables indicating separation distances for the various classes of cables. There are four classification levels of susceptibility for cables. Susceptibility, in this context, is understood to be an indication of how well the signal circuit can differentiate between the undesirable noise and required signal. It follows that a data communication physical standard such as RS-232E would have a high susceptibility and a 1000 volt, 200 amp ac cable has a low susceptibility.

The four susceptibility levels defined by the IEEE 518-1982 standard are briefly:

• Level 1 – High
  This is defined as analog signals less than 50 volt and digital signals less than 15 volt. This would include digital logic buses and telephone circuits. Data communication cables fall into this category.
• Level 2 – Medium
  This category includes analog signals greater than 50 volt and switching circuits.
• Level 3 – Low
  This includes switching signals greater than 50 volt and analog signals greater than 50 volt. Currents less than 20 amp are also included in this category.
• Level 4 – Power
  This includes voltages in the range 0–1000 Volt and currents in the range 20–800 amp. This applies to both ac and dc circuits.

The IEEE 518 also provides for three different situations when calculating the separation distance required between the various levels of susceptibilities.

In considering the specific case where one cable is a high susceptibility cable and the other cable has a varying susceptibility the required separation distance would vary as follows:

• Both cables contained in a separate tray
  • – Level 1 to Level 2 – 30 mm
  • – Level 1 to Level 3 – 160 mm
  • – Level 1 to Level 4 – 670 mm
• One cable contained in a tray and the other in conduit
  • – Level 1 to Level 2 – 30 mm
  • – Level 1 to Level 3 – 110 mm
  • – Level 1 to Level 4 – 460 mm
• Both cables contained in separate conduit
  • – Level 1 to Level 2 – 30 mm
  • – Level 1 to Level 3 – 80 mm
  • – Level 1 to Level 4 – 310 mm

The figures are approximate as the original standard is quoted in inches.

A few words need to be said about the construction of the trays and conduits. It is expected that the trays are manufactured from metal and be firmly earthed with complete continuity throughout the length of the tray. The trays should also be fully covered preventing the possibility of any area being without shielding.

6.9 Earthing and grounding requirements

This is a contentious issue and a detailed discussion laying out all the theory and practice is possibly the only way to minimize the areas of disagreement. The picture is further complicated by the different national codes, which whilst not actively disagreeing with the basic precepts of other countries tend to lay down different practical techniques in the implementation of a good earthing system.

A typical design should be based around two separate electrically insulated earth systems. The two earth systems are:

• The equipment earth
• The instrumentation (and data communications) earth

The aims of these two earthing systems are as follows:

• To minimize the electrical noise in the system
• To reduce the effects of fault or earth loop currents on the instrumentation system
• To minimize the hazardous voltages on equipment due to electrical faults

Earth (or ground) is defined as a common reference point for all signals in equipment situated at zero potential. Below 10 MHz, the principle of a single point earthing system is the optimum solution. Two key concepts to be considered when setting up an effective earthing system are:

• To minimize the effects of impedance coupling between different circuits (i.e. when three different currents for example flow through a common impedance)
• To ensure that earth or ground loops are not created (e.g. by mistakenly tying the screen of a cable at two points to earth)

There are three types of earthing systems possible as shown in Figure 6.17. The series single point is perhaps the more common, while the parallel single point is the preferred approach with a separate earthing system for groups of signals:

• Safety or power earth
• Low level signal (or instrumentation) earth
• High level signal (motor controls) earth
• Building earth

6.10 Suppression techniques

It is often appropriate to approach the problem of electrical noise proactively by limiting the noise at the source. This requires knowledge of the electrical apparatus that is causing the noise and then attempting to reduce the noise caused here. The two main approaches are shown in Figure 6.18.

In Figure 6.18, the inductance will generate a back emf across the contacts when the voltage source applied to it is switched off. This RC network then takes this back emf and thus reduces damage to the contacts.

The voltage can be limited by various combinations of devices (depending on whether the circuit is ac or dc).

The user of these techniques should be aware that the response time of the coil can be reduced by a significant time e.g. the dropout time of a coil can be increased by a factor of ten. Hence, this should be approached with caution where quick response is required from regular switched circuits (apart from the obvious deleterious impact on safety due to slowness of operation).

Two other areas to consider are:

6.11 Filtering

Filtering should be done as close to the source of noise as possible. A table below summarizes some typical sources of noise and possible filtering means.

Asynchronous

In asynchronous communication each character is encoded with a start bit at the beginning of the character bit stream and a parity and stop bit at the end of the character bit stream. The start bit allows the receiver to synchronize with the transmitter so that the receiver looks for each character as it is sent. Once the character has been received the communications link returns to the idle state and the receiver waits for the next start bit indicating the arrival of the next character. This is illustrated in Figure 7.4.

Synchronous communications

Synchronous communication relies on all characters being sent in a continuous bit stream. The first few bytes in the message contain synchronization data allowing the receiver to synchronize to the incoming bit stream. Synchronization is then maintained by a timing signal or clock. The receiver follows the incoming bit stream and maintains a close synchronization between the transmitter clock and receiver clock. Synchronous communication provides for far higher speeds of transmission of data, but is avoided in many systems because of the greater technical complexity of the communications’ hardware.

The major difference between asynchronous and synchronous communications with modems is the need for timing signals.

A synchronous modem outputs a square wave on Pin 15 of the RS-232 DB-25 connector. Pin 15 is called the transmit clock pin or more formally the DCE transmitter signal element timing pin. The square wave is set to the frequency of the modem’s bit rate. The attached personal computer, the DTE, synchronizes its transmission of data from Pin 2 to the modem.

Signal quality detector (CG, Pin 21)

If there is high probability of error in the received data to the modem because of poor signal quality this line is set to OFF.

Data signal rate selector (CH/CI, Pin 23)

If the signal quality detector pin indicates that the quality of the signal is unacceptable, that is, it is set to OFF, the terminal may set Pin 23 to ON to select a higher data rate; or OFF to select a lower data rate. This is called the CH circuit. If, however, the modem selects the data rate and advises the terminal on Pin 23 (ON or OFF), the circuit is known as circuit CI.

ENQ/ACK

The terminal sends an ENQ control character to the modem when it wants to transmit a finite block of data. When the modem is ready to receive characters, it transmits an ACK, which then allows the terminal to commence transmission of this block of data. The process is repeated for subsequent blocks of data.

RTS/CTS signaling

This technique of hardware flow control is a simplified version of the full hardware handshaking sequence discussed. When the terminal wants to transmit data to the modem, it asserts the request to send (RTS) line and waits for the modem to assert the clear to send (CTS) line before transmitting. When the modem is unable to process any further characters it switches off, or inhibits, the CTS control line. The terminal device then stops transmitting characters until the CTS line is again asserted.

Attenuation distortion

Attenuation distortion indicates that the theoretical smooth, horizontal plot of power transmitted versus frequency is not realized in practice. Higher frequencies tend to attenuate more easily and attenuation becomes more non-linear at the edges of the operating bandwidth, or ‘passband’. Hence, the ‘equalizer’ compensates with an equal and opposite effect, giving a constant total loss throughout the passband.

Envelope delay distortion

Envelope delay distortion reflects the reality of transmission of signals down a line where the phase change to frequency is non linear, that is, the phase tends to alter as the signal is transmitted down the communications link. The phase delay is calculated by dividing the phase by the frequency of the signal at any point along the line. The slope of phase versus frequency is called the envelope delay. Delay distortion causes problems in that two different frequencies (indicating a� ‘1’ or a ‘0’ bit) interfere with each other at the receiving modem thus causing a potential error, called intersymbol interference.

Amplitude shift keying (ASK)

The amplitude of the carrier signal is varied according to the binary stream of incoming data. ASK is sometimes still used for low data rates, however, it does have difficulty distinguishing the signal from the noise, as noise in the communications channel is an amplitude based phenomenon.

This form of modulation is indicated in Figure 7.8.

Frequency shift keying (FSK)

Frequency modulation allocates different frequencies to logic 1 and logic 0 of the binary data message. FSK is primarily used by modems operating at data rates of up to 300 bps in full duplex mode and 1200 bps in half duplex mode.

The Bell 103/113 and the compatible ITU V.21 standards are indicated in Table 7.1.

The Bell 103/113 modems are setup in either originate or answer mode. Typically, terminals are connected to originate modems and main frame computers are connected to answer type modems. It is easy to communicate when originate modems are connected to answer mode modems, but similar modems, for example, two originate modems connected together, cannot communicate with each other as they expect different frequencies.

Because of the two different bands of frequencies in which the sets of signals operate, full duplex operation is possible with these modems. Note that they fit into the allowable bandwidth of the communications channel.

Phase shift keying (PSK)

PSK is the process of varying the carrier signal by phase. There are two forms of phase modulation:

• Quadrature phase shift keying (QPSK)
• Differential PSK

QPSK

In QPSK four phase angles are used for encoding:

0°, 90°, 180° and 270°

There are four phase angles possible at any one time, allowing the basic unit of data to be a 2-bit pair, or dibit. The weakness of this approach is that a reference signal is required as indicated in Figure 7.9.

Differential PSK

The preferred option is to use differential PSK where the phase angle for each cycle is calculated relative to the previous cycle as shown in Figure 7.10.

A modulation rate of 600 baud results in a data rate of 1200 bps using two bits for each phase shift.

A typical allocation of dibits, or two bit codes, for each phase shift is as follows:

Quadrature amplitude modulation (QAM)

Two parameters of a sinusoidal signal, amplitude and phase, can be combined to give QAM. QAM allows for 4 bits to be used to encode every amplitude and phase change. Hence, a signal at 2400 baud would provide a data rate of 9600 bps. The first implementation of QAM provided for 12 values of phase angle and 3 values of amplitude.

QAM also uses two carrier signals. The encoder operates on 4 bits for the serial data stream and causes both an in-phase (IP) cosine carrier and a sine wave that serves as the quadrature component (QC) of the signal to be modulated. The transmitted signal is then changed in amplitude and phase resulting in the constellation pattern illustrated above.

Trellis coding

QAM modems are susceptible to noise; hence, a new technique called trellis coding was introduced. Trellis coding allows 9600 to 14 400 bps transmission over normal tele-communication lines and 14 400 bps and higher over good quality leased lines. In order to minimize the errors that occur when noise is evident on the line, an encoder adds a redundant code bit to each symbol interval.

Only certain sequences are valid. If there is noise on the line, which causes the sequence to differ from an accepted sequence, the receiver will select the valid signal point closest to the observed signal without needing a retransmission of the affected data.

A conventional QAM modem, which might require 1 out of every 10 data blocks to be retransmitted, could be replaced by a modem using trellis coding where only one in every 10 000 data blocks might be in error.

Modem transmitter

The modem transmitter contains the following:

• Data encoder
• Scrambler
• Modulator
• Amplifier

Data encoder

The data encoder takes the serial bit stream and uses multilevel encoding, where each signal change represents more than one bit of data, to encode the data. Depending on the modulation technique used the bit rate can be two, four, or more times the baud rate.

Scrambler

The scrambler is used for synchronous operation only. It modifies the bit stream so that long sequences of 1s and 0s do not occur. Long sequences of 1s and 0s are difficult to use in synchronous circuits because of the difficulties they cause in extracting clocking information.

Modulator

The bit stream is converted into the appropriate analogue form using the selected modulation technique. Where initial contact is established with the receiving modem, a carrier is put on the line.

Amplifier

The amplifier increases the level of the signal to the appropriate level for the telephone line and matches the impedance of the line.

Modem receiver

The modem receiver contains the following:

• Filter and amplifier
• Equalizer
• Demodulator
• Descrambler
• Data decoder

Filter and amplifier

Noise is removed from the signal and the resultant signal is amplified.

Equalizer

The equalizer minimizes the effect of attenuation and delay on the various components of the transmitted signal. A predefined modulated signal, called a training signal, is sent down the line by the transmitting modem. The receiving modem knows the ideal characteristics of the training signal and the equalizer will adjust its parameters to correct for the attenuation and delay characteristics of the signal.

Demodulator

The demodulator retrieves the bit stream from the analogue signal.

Descrambler

The descrambler is used in synchronous operation only. The descrambler restores the data to its original serial form after it has been encoded in the scrambler circuit, ensuring that long sequences of 1s and 0s do not occur.

Data decoder

The final bit stream is produced in the data decoder in true RS-232 format.

Dumb modems

Dumb, or non-intelligent, modems depend on the computer to which they are connected to instruct the modem when to perform most of its tasks such as answering the telephone.

Smart modems

Smart modems have an on-board microprocessor enabling them to perform such functions as automatic dialing and selection of the appropriate method of modulation.

As defined by RS-232, any interaction between a traditional dumb modem and the computer equipment occurs by exchanging signal voltages across wires. For example, without pin 20 (DTR) asserted, modems are disabled. However, the smart modem interacts with peripheral equipment by exchanging ASCII character sequences. The smart modem also handles such normally complex tasks as answering the phone automatically, and is capable of answering on a particular ring. A de facto standard has been established based on the Hayes Smartmodem.

The Hayes Smartmodem employs the minimum number of RS-232E functions necessary for full duplex control. RS-232E connections are made through the DB-25S (female) connector.

The smart modem has three states:

• On-line state
• Command state
• Comatose state

On-line state

The on-line state occurs whenever the smart modem is engaged in a carrier link with another modem. In this state, it behaves as a conventional modem transferring all RS-232 input directly to its transmitter.

Command state

When not on-line the smart modem is said to be in the command state and all RS-232 data is treated as a potential command. Usually, modems power up into the command state.

Comatose state

The comatose state is where the DTR pin is inhibited and the modem does not acknowledge commands, participate in dialing activity, or exhibit modem behavior.

Smart modems typically do not use DIP switches to select options because all options and commands are implemented in software. In the command state, the smart modem monitors the bytes incoming from the RS-232 port and watches for a particular sequence of characters referred to as the command sequence introducer. After the smart modem has executed the commands in its command buffer, the smart modem responds with its own sequence of ASCII characters.

There are two general classes of commands:

• Mode commands
• Numeric register commands

Mode commands

There are four basic sets of mode commands:

• User interface group
• Primary answer/dial group
• Answer/dial group
• Miscellaneous

The user interface group commands alter the way in which the smart modem interacts with the user and includes commands, which alter speaker level setting, for example.

The primary answer/dial group commands control the dialing process with commands such as answer, dial, and hook.

The answer/dial group commands affect the characteristics of the primary dial command, the second group with, for example, a pause setting.

The miscellaneous group commands handle such things as managing its own carrier and resetting the modem.

Numeric register commands

The second class of commands is the numeric class which sets up the thirteen status registers (S0 to S12). Other more modern modems use a larger number of registers. There are also three bit mapped registers (S13, S14 and S15). These enable the programmer to query the smart modem about the state of its internal variables, command flags, and current data format.

Status registers

Smart modems use the ATS Command to set and read status registers. A typical selection of S-registers for a smart modem is listed below:

Modes of radio modems

Radio modems can be used in two modes:

• Point to point
• Point to multi-point

A point to point system can operate in continuous RF mode, which has a minimal turn on delay in transmission of data, and non-continuous mode where there is a considerable energy saving. The RTS to CTS delay for continuous and switched carriers is of the order of 10 ms and 20 ms respectively.

A point to multi-point system generally operates with only the master and one radio modem at a time.

In a multi-point system when the data link includes a repeater, data regeneration must be performed to eliminate signal distortion and jitter. Regeneration is not necessary for voice systems where some error is tolerable.

Regeneration is performed by passing the radio signal through the modem which converts the RF analogue signal back to a digital signal and then applies this output binary data stream to the other transmitting modem, which repeats the RF analogue signal to the next location.

Features of a radio modem

Typical features that have to be configured in the radio modem are:

• Transmit/receive radio channel frequency.
  In a point to point configuration running in a dual frequency/split channel assignment, two radios will operate on opposing channel sets.
• Host data rate and format
  Data rate/character size/parity type and number of stop bits for RS-232 communications.
• Radio channel data rate
  Data rate across the radio channel defined by the radio and bandwidth capabilities. Note that these specifications are generally set at the time of manufacture.
• Minimum radio frequency signal level
  Should not be set too low on the receiver otherwise noise data will also be read.
• Supervisory data channel rate
  Used for flow control and should therefore not be set too low otherwise the buffer on the receiver will overflow. Typically one flow control bit to 32 bits of serial data is standard.
• Transmitter key up delay
  The time for the transmitter to energize and stabilize before useful data is sent over the radio link. Transmitter key up delay should be kept as low as possible to minimize overheads.

Spread spectrum radio modems

Several countries around the world have allocated a section of bandwidth for use with spread spectrum radio modems. In Australia and America, this is in the 900 MHz area.

In brief, a very wide band channel is allocated to the modem, for example, approximately 3.6 MHz wide. The transmitter uses a pseudo random code to place individual bits, or groups of bits, broadly across the bandwidth and the receiver uses the same random code to receive them. Because they are random, a number of transceivers can operate on the same channel and a collision of bits will be received as noise by a receiver in close proximity.

The advantage of ‘spread spectrum’ radio modems is very high data security and data speeds of up to 19.2 kbps. The disadvantage is the very inefficient use of the radio spectrum.

MNP protocol classes

MNP defines a system for the detection and correction of errors by retransmission between modems.

There are nine MNP Protocol classes defined in Table 7.3, which cover the transmission alternatives. Smart modems are programmed to attempt an MNP connection at the highest class that both modems can support. An initial frame called the link request is used to establish the standards to be followed in transferring the data. If MNP connection fails, the normal mode is used without error detection, correction, or data compression.

Table 7.3
MNP protocol classes

Link access protocol modem (LAP-M)

This is recognized as the primary method for error detection and correction under the ITU-T V.42 recommendation. MNP error detection and correction is considered the secondary mechanism.

MNP class 5 compression

MNP-5 involves a two stage process:

• Run length encoding
• Adaptive frequency encoding

Run length encoding

The first three bytes indicate the beginning of a run length encoded sequence. The next byte is the repetition count of bits, with a maximum number of 250 bits. For runs of similar bits, this can reduce the total size of the data bytes dramatically.

Essentially, the number of successive bits, which are the same, are counted and then coded into an eight bit symbol, for example. The eight-bit symbol is then transmitted.

Data compression is used extensively in the fax machine. For example in Group 3 machines, a regular 11 inch sheet of paper can be vertically digitized into 100 lines per inch to produce 1100 lines and horizontally each line is further digitized into 1700 bits/line.

Total size of the file = 1700 bits/line × 1100 lines = 1.87 Mbits

Assuming this file is sent on a 2400 baud modem, the transmission time for one page of text would be 779 seconds, as calculated below. However, in practice, the transmission time of a page is about 30 to 60 seconds. Data compression is used to achieve these results.

The microprocessor on the facsimile machine can process the data bits before sending them and uses a compression algorithm for compressing the data into fewer bits.

Adaptive frequency coding

In adaptive frequency coding a compression token is substituted for the actual byte transferred. Shorter tokens are substituted for more frequently occurring data bytes. A compression token consists of two parts:

• A fixed length header, 3 bits long, which indicates the length of the body
• A variable length body

At compression initialization, a table is set up for each byte from 0 to 255. To encode a data byte, the token to which it is mapped is substituted for the actual data byte in the data stream. The frequency of occurrence of the current data byte is increased incrementally by one. If the frequency of occurrence of the current data byte is greater than the frequency of the next most frequently occurring data byte, the two tokens are swapped. This comparison process is repeated for the next most frequently occurring data byte and the tokens are again swapped.

MNP class 7: enhanced data compression

MNP class 7 combines run length encoding with an adaptive encoding table. The table is used to predict the probability of a character occurring, based on the value of the previous character. Up to 256 (28) coding tables are kept for each 8-bit pattern. All characters are organized according to the rules of Huffman coding.

Huffman encoding

Huffman encoding relies on some characters occurring more frequently than others. The Huffman code is computed by determining the frequency of occurrence of each symbol in the set of symbols used for communications.

The following steps should be followed in computing the Huffman codes:

• List next to each of the symbols used the probability of occurrence in the message. The sum total of probabilities must total 1. For example, the symbols A, X, Y, S are used with probabilities of occurrence indicated in brackets as follows: A(0.2); X(0.1); Y(0.4); Z(0.3) Note: The sum totals of the probabilities must equal one.
• Write the symbols in order of ascending probability of occurrence.

Add the two lowest probabilities and form a new node over the two nodes with the sum of the probabilities as in Figure 7.19.

• Repeat the process with the new node created and the next node to the left in order of probability.
• Repeat this process until completed, with a result as indicated in Figure 7.20.

• Assign a 1 to the branches leaning in one direction as indicated above and 0 to the remaining branches.
• Compute the Huffman code for each symbol by tracing the path from the apex of the pyramid to each base.

Hence:

Y = 0
Z = 10
A = 110
X = 111

In order to compute the compression ratio, as compared to the standard 7-bit ASCII code, assume there are 1000 symbols (that is, Y, Z, A and X) transmitted.

Total bits using Huffman encoding =
(Probability of occurrence of symbol of 0.4 * 1000 symbols) * 1 bit/symbol Y +
(Probability of occurrence of symbol of 0.3 * 1000 symbols) * 2 bits/symbol Z +
(Probability of occurrence of symbol of 0.2 * 1000 symbols) * 3 bits/symbol A +
(Probability of occurrence of symbol of 0.1 * 1000 symbols) * 3 bits/symbol X
= 400 + 600 + 600 + 300
= 1900 bits

If the ASCII code had been used this would have resulted in –

Once the Huffman code has been computed, the software converts each symbol into its equivalent code and includes the table used for translating the code back to symbols in the original transmission. The receiver software will then decompress the stream of bits into the original stream of symbols.

Run length encoding is used if there are four or more identical characters in a specified sequence of characters. The first three characters are encoded (as for the rules of Huffman encoding) and the number of remaining, identical characters is encoded in a 4-bit nibble.

Decoding the data stream is achieved quite simply because the receiving modem keeps the same compression table as the transmitting modem.

V.42bis

V.42bis relies on the construction of a dictionary, which is continually modified as data is transferred between two modems. The dictionary consists of a set of trees in which each root corresponds to a character in the alphabet. When communications is established each tree comprises a root node with a unique code word assigned to each node. The sequence of characters received by the modem from its attached terminal is compared with and matched against the dictionary.

The maximum string length can vary from 6 to 250 characters and is defined by the two connecting modems. The number of code words has a minimum of 512, but any value above this default minimum value can be agreed between the two connecting modems.

V.42bis data compression, in substituting a code word for a string, is between twenty and thirty per cent more efficient than MNP class 5 compression. V.42bis is effective for large file transfers, but not for short strings of data.

Troubleshooting the modem

There are various tests available for troubleshooting operational problems associated with a modem, which fall into two categories:

• Self test
• Loop back test

Self Test

The self test is where the modem connects its transmitter to its receiver. The connection with the communications line is broken and a specified sequence of bits is transmitted to the receiving parts of the modem where this is then compared with a defined pattern. An error will be indicated on the modem front panel if the transmitted sequence does not match the expected pattern.

Loop back tests

The second set of tests is the loop back tests. There are four forms of loop back tests:

• Local digital loop to test the terminal or computer and connecting RS-232 line
• Local analog loop to test the modem’s modulator and demodulator circuitry
• Remote analog loop to test the connecting cable and local modem
• Remote digital loop to test the local and remote modem and connecting cable

Space division multiplexing (SDM)

SDM is where multiple paths are created by running new physical channels next to the existing ones to connect a receiver and transmitter as shown in Figure 7.24. Some authorities feel that SDM is not a true multiplexing method. The technique is generally considered unattractive because additional cables, transmitters, and receivers are required.

The best example of space division multiplexing is the local telephone system. Each telephone is connected to the central office by a local loop not shared by other subscribers.

Demultiplexing SDM systems is virtually unnecessary as each signal has its own independent link and receiver/transmitter equipment.

Frequency division multiplexing (FDM)

FDM is where different, unique frequencies are used by each channel enabling several channels to use the same medium electrical cable. FDM occurs, therefore, when the bandwidth of the link is greater than the bandwidth of the messages sent over the link.

FDM is used extensively in telemetry and radio/TV broadcast applications where each sensor signal, representing for example, temperature, pressure and speed, is within a 0–1 V range, suitable for narrow band FM, and each has a final bandwidth of 4000 Hz. A basic signal of 0–4 kHz is called the ‘baseband’ signal. All of these baseband signals are multiplexed with various sub-carriers spaced 4 kHz apart with a bandwidth extending from 0 Hz to 4 kHz times the number of telemetry channels. Figure 7.26 gives an example of the division of a frequency spectrum.

Care should be taken to maximize the available total bandwidth. For example, using a 4 MHz bandwidth spacing to send 4 kHz bandwidth signals wastes (4000–4) kHz = 3996 kHz, using only 7% of the spectrum.

At the receiver, demultiplexing begins with translation of the multiplexed signal down to an intermediate frequency, with a local oscillator and mixer for each channel. The intermediate frequency bandwidth is set equal to the bandwidth of an individual baseband signal after modulation. A receiver that demultiplexes ten signals requires ten separate local oscillators and mixers, with each oscillator operating at the frequency appropriate for its intended signal.

Figure 7.27 is a block diagram of a basic FDM transmitter and receiver circuit.

Time division multiplexing (TDM)

In a communications system, it is possible for the many users to time share the physical links by switching each signal for a short period of time, as in Figure 7.28. As the scanning rate rises, the system will eventually become ineffective because of an increase in the following:

• Propagation delays
• Noise
• Errors
• Retransmission

Unlike analog signals, digital TDM has greater latitude in sampling each bit of each channel. As long as samples occur within the bit period, even though it may be late or early in the specific bit period, no data will be lost. The greatest limitation of TDM lies in the bandwidth of the communications medium. As the bit rate increases, the frequency requirements of that medium also increase.

Practical example

The Modbus Plus bridge/multiplexer is a practical example of an alternative use of a multiplexer in TDM mode. This permits the connection of Modbus based devices to Modbus Plus networks. The example illustrates the use of a multiplexer in a broader sense, rather than maximizing the use of a communication channel. Figure 7.29 shows the NW-BM85-000 Modbus Plus bridge/multiplexer from AEG Modicon.

This multiplexer permits interconnection of four Modbus ports and one Modbus Plus port. Configuration is possible by setting a few switches and doing some software configuration, which is then stored in an EEPROM. A buffer in the multiplexer allows up to 512 transactions to be queued. The software automatically connects the relevant port to its destination. Alternatively, it can be used as a four channel modbus port expander permitting connection of master devices to either individual slaves or several networks of modbus slave devices.

Objectives

When you have completed studying this chapter you will be able to:

• Define the term ‘protocol’
• Describe the role of protocols in flow control
• Describe the two most popular types of flow control protocols:
  • – XON/XOFF
  • – ETX/ACK
• Differentiate between and describe the two modes of binary synchronous protocols:
  • – Point-to-point
  • – Multipoint
• Describe HDLC and SDLC protocols in terms of:
  • – Frame format
  • – Frame contents
  • – Operation
  • – Error/flow control
• Describe file transfer protocols
• Describe ARQ protocols

As we will see in Chapter 9, data communications systems, which follow the OSI reference model, are made up of several hierarchical layers. Each of these layers contains working software or hardware elements referred to as entities. One of the elements in each layer is a protocol entity, which has its own specification, and broadly speaking is a protocol in its own right. The purpose of the protocol entity is to determine how messages are transferred across a network to a peer entity in another node.

Actual transfers across the physical link are determined by the data link layer protocol, and it is these types of protocols that are discussed in this chapter. Other more complex protocols are discussed in later chapters.

A protocol is concerned with some or all of the following:

• Initialization, to start the transmission of data.
• Framing and frame synchronization – this defines the beginning and end of a frame and ensures that the receiver can synchronize with the frame.
• Flow control, to ensure that the receiver is not swamped.
• Line control, applies to half duplex links in which the transmitter tells the receiver to start transmitting.
• Error control.
• Timeout control, so action can be taken if no acknowledgment is received within a certain period.

8.1 Flow control protocols

The most elementary protocols are only concerned with flow control and were introduced as an improvement on simple techniques such as the insertion of delays between characters or echoing of received characters to the transmitter. The two most popular types of flow control protocols are XON/XOFF and ETX/ACK.

8.2 XON/OFF

This is a character based flow-control protocol, which uses two special characters. Typically, these are the ASCII characters DC1 for XON and DC3 for XOFF. The transmitter sends data until it receives an XOFF from the receiver; it then waits for an XON before resuming transmission. A typical example can be found in a printer buffer. When the buffer reaches a certain point (say 66%), the printer sends an XOFF to the PC, then sends an XON when the buffer is emptied to another point (say 33%).

One disadvantage of XON/XOFF is that the data stream being sent may contain one of the control characters, although this is not a problem in applications such as printer control.

8.3 Binary synchronous protocol

The binary synchronous control (BSC) protocol was designed by IBM in 1966 for computer-to-terminal and computer-to-computer communications. It can be used in point-to-point or in multipoint mode. BSC is a character-based protocol as opposed to the high-level data link control (HDLC) protocol, which is bit-based. HDLC is discussed under ‘HDLC and SDLC protocols’ below.

The XON/XOFF flow control mechanism can easily handle short interactive messages between a terminal and a computer. They are less adequate in ‘block mode’, that is when passing complete messages, with hundreds or even thousands of characters, between terminals. The BSC protocol, on the other hand, was specifically designed to handle large blocks of data.

Control characters are used to separate the different fields in a BSC message and for the exchange of acknowledgment information.

Table 8.1 lists the control characters involved in BSC.

There are various types of BSC messages, as given in Figure 8.1.

The receiver of the message uses the two SYN characters, (bit pattern 0010110) to synchronize with the start of the message. Note that the SYN characters are not considered to be part of the message itself and are therefore not used in the calculation of the block check character (BCC). In order to maintain synchronization, the transmitter inserts SYN characters into text messages once every second; however, as before, these are not used in BCC calculations.

A text field starts with the STX character and ends with ETX, ETB, EOT or ITB, as appropriate. The BCC field consists of a vertical/longitudinal check or a cyclic redundancy check such as the CRC-16 used for transparent mode (see Data Transparency later in this chapter).

If a message is received without error, the receiver responds with ACK1 for the first response, ACK2 for the next, and so on, alternating at each response so that odd numbered messages return ACK1 and even numbered messages return ACK2. This enables the transmitter to track the responses and detect any messages not acknowledged. ACK1 is represented by the sequence DLE00, and ACK2 is represented by the sequence DLE01.

If the receiver detects an error in a message, it responds with a NAK.
Point-to-point mode full sequence transmission is as follows:

8.4 HDLC and SDLC protocols

HDLC has been defined by the International Standards Organization for use on both multipoint and point-to-point links. Other descriptions of it include SDLC (synchronous data link control used by IBM) and ADCCP (advanced data communication control procedure used by ANSI). HDLC will be the reference used throughout the following text. In contrast to the BSC protocol, HDLC is a bit-based protocol. It is interesting to note that it is a predecessor to the local area network datalink protocols.

The two most common modes of operation for HDLC are:

• Unbalanced normal response mode (NRM). This is used with only one primary (or master) station initiating all transactions.
• Asynchronous balanced mode (ABM). In this mode each node has equal status and can act as either a primary or secondary node.

Frame format

The standard format is indicated Figure 8.2. The three different classes of frames used are as follows:

Unnumbered frames:	Used for setting up the link or connection and to define whether NRM or ABM is to be used. They are called unnumbered frames because no sequence numbers are included.
Information frames:	Used to convey the actual data from one node to another.
Supervisory frames:	Used for flow control and error control purposes. They indicate whether the secondary station is available to receive the information frames; they are also used to acknowledge the frames. There are two forms of error control used: a selective retransmission procedure because of an error, or a request to transmit a number of previous frames.

8.5 File transfer protocols

In most asynchronous file transfers used on PCs, the basic structure is the packet (or frame) consisting of a group of fields. Only one of these fields contains the actual data. The remaining fields, known as service fields, contain the information required for the receiver to verify that the packet is error free.

Automatic repeat request (ARQ) protocols

The most common type of packet protocol is the automatic repeat request (ARQ) protocol in which an error detected in a received packet and an unacknowledged packet automatically results in the retransmission of the packet.

Send and wait ARQ

Here the receiver inputs the packets and after verifying that the packet is in the correct sequence relative to the previous packet, computes a local check value on the data portion of the packet. On a successful match with the one in the packet the receiver acknowledges with an ACK; or sends a NAK. When the transmitter receives the ACK it then sends the next packet.

Continuous ARQ

Here the transmitter sends several packets in a row with no delay between packets. The receiver sends a NAK or ACK (as per the send and wait ARQ) together with the packet’s number. The transmitter continually examines the stream of acknowledgments returning and keeps track of the packets with errors. At the end of the transmission the packets with errors are retransmitted.

The send and wait ARQ remains the most popular file transfer protocol found on PCs.

Packet design

There are three approaches for design of packets as shown in the figures below.

The two types of protocols that are used for file transfer are XMODEM and Kermit.

XMODEM

XMODEM is a simple send and wait ARQ protocol that uses a fixed length data field. The check value is a single byte arithmetic checksum.

Layer 7: The application layer

The application layer is the top most layer in the OSI/RM. It is responsible for giving applications access to the network. Examples of application layer tasks include file transfer, e-mail services and network management.

To accomplish the tasks, the application layer passes program requests and data to the presentation layer, which is responsible for encoding the application layer’s data in the appropriate form.

Layer 6: The presentation layer

This layer maps the data representations into a suitable format for the recipient. It translates the format and syntax of the data produced by applications, and handles encryption or compression of the data.

Layer 5: The session layer

This layer manages interactive sessions between devices. It defines the use of software that allows reference to other devices by name rather than binary address (logical naming). It allows seamless recovery of sessions.

Layer 4: The transport layer

This layer defines:

• The management of the communications between two end systems.
• Data transfer at agreed levels of quality.
• How the packets in a large message are accounted for and correctly segmented. This improves the reliability of data transfer.

Layer 3: The network layer

This layer defines:

• How the packets of information are routed around the network.
• How the status messages are regulated and sent to other devices on the network.
• How large packets of data received from the transport layer are fragmented into small ones.
• Enough frames to go through the underlying network.

Layer 2: The data link layer

This layer is always needed and defines:

• The methods used to access the network for transmitting and receiving messages.
• The handling of the information received and its acknowledgment.
• The procedures for managing flow control between users.

Layer 1: The physical layer

This layer is always needed and defines the physical connections between the computer and the network. It provides Layer 2, the Data Link Layer, with the physical means of transmitting data onto the network.

This layer is concerned with the following:

• The network topology
• The electrical aspects of voltages and currents
• The signal modulation techniques
• The mechanical aspects of the connection, i.e. the cables and connectors

Note: It stipulates, but does not include the actual medium.

Objectives

When you have completed study of this chapter you will be able to:

• Describe the features of industrial protocols
• Describe the use of ASCII based protocols
• Describe the read and write commands of ANSI-X3.28-2.5-A4
• List and describe the three Modbus structures
• Describe the Modbus protocol:
  • – Message format
  • – Synchronization
  • – Memory location
  • – Function codes
  • – Exception responses
• Describe the Allen Bradley Data Highway protocol
• Describe the Allen Bradley Data Highway Plus protocol
• Describe the OSI model layers used by the Allen Bradley Data Highway Plus protocol
• Describe the application of MAP/TOP protocols

10.1 Introduction

In some respects the distinction between an industrial and commercial (or data processing) protocol is somewhat artificial. There are, however, a few features contained in an industrial protocol, which can make it useful to an engineer on a plant.

These are:

• Ease of troubleshooting systems
  Where the level of understanding of industrial communication systems on a plant may be fairly low it makes sense to select a simple protocol such as one of the ASCII protocols.
• High level of integrity of data transfer
  In an industrial environment where there is electrical noise and no errors in the transfer of the data are acceptable (due to the communications link controlling critical equipment for example), a protocol should be selected with a high degree of error checking such as cyclic redundancy checks.
• Standardization of protocol
  There may be a requirement to interface to other manufacturers PLCs or industrial systems. In this case a commonly accepted industrial protocol such as Modbus would be appropriate.
• High speed update of parameters
  There may be a necessity to update a number of setpoints to a series of controlling devices virtually simultaneously. Here one of the new FieldBus protocols may be appropriate to ensure that there is no skew (or delays) between transferring the setpoint to the first and last devices on the data highway.

10.2 ASCII based protocols

ASCII based protocols are popular because of their simplicity. Their main disadvantage is that they are slow and become unwieldy for larger systems with the requirement for multiple nodes on a network that need to communicate with one another (rather than in a simple one master, multiple slave setup).

Consequently, ASCII based protocols are normally only used for slow systems with one master talking to a limited number of slaves (preferably only one).

ASCII based protocols are also popular for stand-alone instruments where a serial interface has been added on with no major design changes to the existing design. Essentially this means that the additional serial port is treated like another keypad by the instrument.

Although ASCII based protocols would appear to be the simplest; in the author’s experience they have proved problematic in their implementation because of the lack of a tight definition by the particular manufacturer.

Two ASCII based protocol implementations are given below. The first is for the implementation of smart transmitters and the other for a variable speed drive. The smart transmitter is a fairly simple protocol structure whilst the ANSI X3.28 2.5-A4 is a slightly more complex approach.

ASCII based protocols for digital transmitters

A variety of digital signal transmitters has recently appeared on the market, which accept a variety of sensor and process inputs and communicate the data back to the serial port of a computer or other processor based device in a digital format. Data is also sent down from the computer to the signal transmitter for controlling devices (via a digital or analog output from the conditioner unit). The RS-232 or RS-485 standards are used for communications between the signal transmitter and computer.

Each digital transmitter is a complete single channel interface system with analog signal conditioning electronics optimized for a specific input type. The analog input signals are digitized by an analog to digital (A/D) converter whilst the analog output signals are converted from their digital form by a digital to analog converter. All data is stored in an ASCII format in a buffer where contents can be updated about eight times per second. The host computer may transmit or request data from or to the transmitter by sending simple ASCII commands to it.

There are many variations on the standard transmitter available such as for high frequency inputs, digital inputs and outputs, thermocouple and RTD (resistance, temperature, dependent) inputs and analog outputs.

Communications hardware

The RS-232 standard is used as a point-to-point communication system but these transmitters can be setup with multiple units ‘hanging off’ the same RS-232 communications port. However, as the RS-232 standard does not allow a multi-node system, the units are daisy chained as indicated in the Figure 10.1. In this network any characters transmitted by the host computer are received by each transmitter in the chain and passed on to the next until a transmitter recognizes its address and then transmits a response which is rippled back through other transmitters in the chain.

The RS-485 standard is used in a half duplex fashion for multidropped systems. If more than 32 modules are required to hang off the same RS-485 port, then an RS-485 repeater module is required to boost the signal and supply power to the additional modules.

The transmitter module contains an EEPROM (electrically erasable programmable read only memory) to store setup information and calibration constants. As the communications parameters (such as baud rate) could be forgotten by the user, the module can be put into a default mode where it resets to 300 baud, no parity and recognizes any address.

Protocol structure

A simple command/response ASCII based protocol is used for communications between the host computer and the transmitter module. The host computer always generates the command sequence.

Communications is performed with two character ASCII command codes. All analog data is requested as a nine-character string consisting of a sign, five digits, decimal point and two additional digits.

A typical command/response is indicated in Figure 10.3.

This command reads from transmitter at address 1 and receives a value of 275.00 in the response message.

The maximum length of the command and response messages is 20 printable characters (i.e. non ASCII control characters).

A variation on the short form command and response messages are the long form which is used to ensure greater response message integrity and echoes the command message and appends a block checksum at the end of the message. The long form command is initiated using a # in place of the $ signifying the commencement of the command message. Note that the two-character checksum can be optionally added to all command messages at the host computer’s discretion.

A checksum is simply the sum of the hexadecimal values of all the ASCII characters in the message.

Note: The calculation of the checksum for the response is performed as follows:

Discard the 2 and add the A4 onto the end of the message.

Note that the A and 4 being hexadecimal characters have to be sent as their ASCII equivalents.

Errors

If the transmitter module indicates that it has received a message with an error in it, it will respond with ‘?’ character. Alternatively, there may be no response at all if an incorrect address or command prompt has been used.

Typical error responses are illustrated in Figure 10.5.

10.3 ASCII based protocol ANSI-X3.28-2.5-A4

ANSI X3.28 2.5-A4 is an example of an ASCII based protocol. One particular manufacturer of variable speed drives, Control Techniques, uses this protocol to communicate from a programmable controller or personal computer to up to 32 drives. Generally the RS-485 standard is preferred in the implementation of this protocol.

Overall approach

The ANSI X3.28 2.5-A4 standard defines the character format and sequence of characters in a message. The typical structure adopted in the RS-485 standard is:

• A 10 bit character consisting of 1 start bit, 7 bit ASCII Code, even or no parity, 1 stop bit
• Baud rate selectable between 300 and 19 200 bps
• Up to 32 drives or slaves permitted on the network

There are two types of commands:

• READ command from the PC to the specific drive requesting information on a specific parameter
• A WRITE command from the PC to the specific drive to change READ/WRITE parameters

The read command

The read command and its response have the format shown in Figure 10.6.

The ASCII characters used in the READ request message are:

• EOT (1 character)
  This resets all the devices connected to the serial link
• ADD (4 characters)
  The protocol allows addressing up to 32 devices. Each character in the address is transmitted twice, to ensure data integrity. For example if drive address is 14, ADD = 1144
• PAR (3 characters)
  This consists of the parameter information, which is in the range of 0–999. For bit parameters, add 100 to the bit number
• ENQ (1 character)
  Termination of message

The ASCII characters used in the READ response message are:

• STX (1 character)
  This indicates to the master the start of the reply
• PAR (3 characters)
  As above
• DATA (6 characters)
  The first character consists of the polarity (or a space if this is irrelevant). A maximum of four digits and a decimal point form the remaining characters.
• ETX (1 character)
  This indicates to the master that the data is finished.
• BCC (1 character)
  The block checksum forms the error checking mechanism. This is calculated by doing an exclusive OR calculation on the parameter number, the data and the ASCII character EOT. Sometimes the BCC is disabled on the equipment and a CR is only returned.

The message interchange between the master and slave can be continued after the first READ request and response frames by:

• Stepping forward to the next (sequential) parameter by sending an ACK

or

• Stepping backwards to the previous (sequential) parameter by sending a BS

or

• Requesting the same parameter again by sending a NAK to the slave

This can be repeated indefinitely (until the end of the parameter list). Sending of EOT resets all the devices on the network.

The write command

The same descriptions as for the READ command apply. In addition the slave will return the NAK character if the drive parameter, data or BCC is in error.

If it is requested to write further data to the drive, the sequence outlined in Figure 10.9 should be followed.

10.4 Modbus protocol

Protocol specifics

This section reviews the Modbus protocol in detail and is broken down into the following sections:

• Message format
• Synchronization
• Memory location
• Function codes
• Exception responses

Message format

A transaction consists of a single request from the host to a specific secondary device and a single response from that device back to the host. Both of these messages are formatted as Modbus message frames. Each such message frame consists of a series of bytes grouped into four fields as described in the following paragraphs. Note that each of these bytes indicated here are in Hex format (not ASCII).

The first field in each message frame is the address field, which consists of a single byte of information. In request frames, this byte identifies the controller to which the request is being directed. The resulting response frame begins with the address of the responding device. Each slave can have an address field between 1 and 247, although practical limitations will limit the maximum number of slaves. A typical Modbus installation will have one master and two or three slaves.

The second field in each message is the function field, which also consists of a single byte of information. In a host request, this byte identifies the function which the target PLC is to perform.

If the target PLC is able to perform the requested function, the function field of its response will echo that of the original request. Otherwise, the function field of the request will be echoed with its most-significant bit set to one, thus signaling an exception response. Table 10.2 summarizes the typical functions used.

The third field in a message frame is the data field, which varies in length according to which function is specified in the function field. In a host request, this field contains information the PLC may need to complete the requested function. In a PLC response, this field contains any data requested by that host.

The last two bytes in a message frame comprise the error-check field. The numeric value of this field is calculated by performing a cyclic redundancy check (CRC-16) on the message frame. This error checking ensures that devices do not react to messages that may have been changed during transmission.

Synchronization

In order to achieve reliable communication, the reception of a message must be synchronized with its transmission. In other words, the receiving device must be able to identify the start of a new message frame. Under the Modbus RTU protocol, frame synchronization is established by limiting the idle time between successive characters within a message frame. If three character times (approximately three milliseconds) elapse without the receiving device detecting a new character, the pending message will be flushed. The next byte will then be interpreted as the address field of a new message line.

Memory notation

The memory notation allows for four different data types: coils, discrete inputs, input registers and holding registers. Register variables consist of two bytes, while coils and discrete inputs are single bytes.

Each function references only one type of data. This allows message-frame memory references to be expressed as hexadecimal offsets relative to the lowest possible address for that data type. For example, the first holding register (40001) is referenced as 0000.

Table 10.2 lists the address range and offsets for these four data types, as well as the function codes, which apply to each. The diagram above also gives an easy reference to the Modbus data types.

Function codes

Each request frame contains a function code that defines the action expected for the target controller. The meaning of the request data fields is dependent on the function code specified.

The following paragraphs define and illustrate most of the popular function codes supported. In these examples, the contents of the message-frame fields are shown as hexadecimal bytes.

Read coil or digital output status (function code 01)

This function allows the host to obtain the ON/OFF status of one or more logic coils in the target device.

The data field of the request consists of the relative address of the first coil followed by the number of coils to be read. The data field of the response frame consists of a count of the coil bytes followed by that many bytes of coil data.

The coil data bytes are packed with one bit for the status of each consecutive coil (1=ON, 0=OFF). The least significant bit of the first coil data byte conveys the status of the first coil read. IF the number of coils read is not an even multiple of eight, the last data byte will be padded with zeros on the high end. Note that if multiple data bytes are requested, the low order bit of the first data byte in the response of the slave contains the first addressed coil.

In the following example, the host requests the status of coils 000A (decimal 00011) and 000B (00012). The target device’s response indicates both coils are ON.

This function enables the host to read one or more discrete inputs in the target device.

The data field of the request frame consists of the relative address of the first discrete input followed by the number of discrete inputs to be read. The data field of the response frame consists of a count of the discrete input data bytes followed by that many bytes of discrete input data.

The discrete-input data bytes are packed with one bit for the status of each consecutive discrete input (1=ON, 0=OFF). The least significant bit of the first discrete input data byte conveys the status of the first input read. If the number of discrete inputs read is not an even multiple of eight, the last data byte will be padded with zeros on the high end. The low order bit of the first byte of the response from the slave contains the first addressed digital input.

In the following example, the host requests the status of discrete inputs hexadecimal offsets 0000 and 0001 (i.e. decimal 10001 and 10002). The target device’s response indicates that discrete input 10001 is OFF and 10002 is ON.

Read holding registers (function code 03)

This function allows the host to obtain the contents of one or more holding registers in the target device.

The data field of the request frame consists of the relative address of the first holding register followed by the number of registers to be read. The data field of the response time consists of a count of the register data bytes followed by that many bytes of holding register data.

The contents, of each requested register, are returned in two consecutive register-data bytes (most significant byte first).

In the following example, the host requests the contents of holding register hexadecimal offset 0002 or decimal 40003. The controller’s response indicates that the numerical value of the register’s contents is hexadecimal 07FF or decimal 2047. The first byte of the response register data is the high order byte of the first addressed register.

Reading input registers (function code 04)

This function allows the host to obtain the contents of one or more input registers in the target device.

The data field of the request frame consists of the relative address of the first input register followed by the number of registers to be read. The data field of the response frame consists of a count of the register-data bytes followed by that many bytes of input-register data.

The contents, of each requested register, are returned in two consecutive register-data bytes (most-significant byte first). The range for register variables is 0 to 4095.

In the following example, the host requests the contents of input register hexadecimal offset 000 or decimal 30001. The PLC’s response indicates that the numerical value of that register’s contents is 03FFH, which would correspond to a data value of 25 per cent (if the scaling of 0 to 100 per cent is adopted) and a 12-bit A/D converter with a numerical range of 0 to 4095 (0FFFH) is used.

Force single coil (function code 05)

This function allows the host to alter the ON/OFF status of a single logic coil in the target device.

The data field of the request frame consists of the relative address of the coil followed by the desired status for that coil. A hexadecimal status value of FF00 will activate the coil, while a status value of zero (H) will deactivate it. Any other status value is illegal.

If the controller is able to force the specified coil to the Requested state, the response frame will be identical to the request. Otherwise an exception response will be returned.

If the address 00 is used to indicate broadcast mode, all attached slaves will modify the specified coil address to the state required.

The following example illustrates a successful attempt to force coil 11 (decimal) OFF.

Preset single register (function code 06)

This function enables the host to alter the contents of a single holding register in the target device.

The data field of the request frame consists of the relative address of the holding register followed by the new value to be written to that register (most-significant byte first).

If the controller is able to write the requested new value to the specified register, the response frame will be identical to the request. Otherwise, an exception response will be returned.

The following example illustrates a successful attempt to change the contents of holding register 40003 to 3072 (0C00 Hex).

When slave address is set to 00 (broadcast mode), all slaves will load the specified register with the value specified.

Read exception status (function code 07)

This is a short message requesting the status of eight digital points within the slave device.

This will provide the status of eight predefined digital points in the slave. For example this could be items such as the status of the battery, whether memory protect has been enabled or the status of the remote input/output racks connected to the system.

Loopback test (function code 08)

The objective of this function code is to test the operation of the communications system without affecting the memory tables of the slave device. It is also possible to implement additional diagnostic features in a slave device (should this be considered necessary) such as number of CRC errors, number of exception reports etc.

The most common implementation will only be considered in this section; namely a simple return of the query message.

Force multiple coils or digital outputs (function code 0F)

This forces a contiguous (or adjacent) group of coils to an ON or OFF state. The following example sets 10 coils starting at address 01 Hex (at slave address 01) to the ON state. If slave address 00 is used in the request frame broadcast mode will be implemented resulting in all slaves changing their coils at the defined addresses.

Force multiple registers (function code 10)

This is similar to the preset single register and the forcing of multiple coils. In the example below, a slave address 01 has 2 registers changed commencing at address 4011.

Exception responses

Request frames containing parity or checksum errors are ignored – no response is sent by any device. If an otherwise valid request frame contains an illegal request (one not supported by the target slave unit), an exception response will be returned to the host.

The four fields of an exception response contain:

• The address of the responding controller
• The requested function number with its most-significant bit set to one
• An appropriate exception code
• The CRC-16 checksum

Table 10.3 lists the most important exception codes, which may be returned.

An example of an illegal request and the corresponding exception response is shown below. The request in this example is to READ COIL STATUS of points 514 to 521 (eight coils beginning an offset 0201H). These points are not supported in this PLC, so an exception report is generated indicating code 02, illegal address.

10.5 Allen Bradley Data Highway (Plus) protocol

Overview of Allen Bradley protocol

There are two main protocol standards used in Allen Bradley data communications:

The Data Highway protocol

This is a local area network (LAN) that allows peer-to-peer communications up to 64 nodes. It uses a half duplex (polled) protocol and rotation of link mastership. It operates at 57.6 kBaud.

The Data Highway Plus protocol

This is similar to the Data Highway Network although designed for fewer PCs and operates at a data rate of 57.6 kbaud. This has peer-to-peer communications with a token passing scheme to rotate link mastership among the nodes connected to that link.

Note that both protocol standards implement peer-to-peer communications through a modified token passing system called the floating master. This is a fairly efficient mechanism as each node has an opportunity to become a master at which time it can immediately transmit without checking with each mode for the requisite permission to commence transmission.

The Allen Bradley Data Highway Plus uses the three layers of the OSI layer model:

• Hardware (a physical layer)
• Data link layer protocol
• Application layer protocol

Physical layer (hardware layer)

This is based on twin axial cable with three conductors essentially in line with the RS-485 specifications.

Full duplex data link layer

Note that the asynchronous link can use either a full duplex (unpolled) protocol or a master slave communication through a half duplex (unpolled) protocol. Although both types of protocols are available the tendency today is to use the full duplex protocol as this explains the high performance nature of the link. Hence this protocol will be examined in more detail in the following sections.

Full duplex protocol is character orientated. It uses the ASCII control characters listed in the following table, extended to eight bits by adding a zero for bit number seven (i.e. the eighth bit).

The following ASCII characters are used:

Full duplex protocol combines these characters into control and data symbols. The following table lists the symbols used for full duplex implementation.

Format of a message

Note that response symbols transmitted within a message packet are referred to as embedded responses.

The CRC-16 calculation is done using the value of the application layer data bytes and the ETX byte. The CRC-16 result consists of two bytes. Refer to section 8.4 for more information on the cyclic redundancy check mechanism.

Note that to transmit the data value of 10H, the sequence of data symbols DLE DLE must be used. Only one of these DLE bytes and no embedded responses are included in the CRC value.

Message limitations

• Minimum size of a valid message is six bytes.
• Duplicate message detection algorithm – receiver compares the second, third, fifth and sixth bytes of a message with the same bytes in the previous message.

P = Recovery procedure
T = Ready to transmit next message
* = Default values used by module

Depending on the highway traffic and saturation level, there may be a wait for a reply from the remote node before transmitting the next message.

 

Software logic for receivers

The following diagrams show typical events that occur in the communications process.

There are two types of application programs:

• Command initiators
• Command executors

Command initiators specify which command function to execute at a particular remote node.

The command executor must also issue a reply message for each command it receives. If the executor cannot execute a command it must send the appropriate error code.

The reply message may contain an error. The command initiator must check for this condition and depending on the type of error, retransmit the message or notify the user.

If the command executor reply is lost due to noise, the command initiator should maintain a timer for each outstanding command message. If the timer expires the command initiator should take appropriate action (notify the user or retransmit to executor).

If the application layer software cannot deliver a command message, it should generate a reply message with the appropriate error code and send the reply to the initiator. If it cannot deliver a reply message, the application layer should destroy the reply without notification to the command executor.

Note that not all command messages have FNC, ADR, SIZE or DATA bytes. Not all reply messages have DATA or EXT STS bytes.

Explanation of Bytes

DST	Destination byte. This contains the ultimate destination of the node.
SRC	Source node of the message. Set this to zero as the KF2 interface module will set the byte to its own node number.
CMD	Command byte.
FNC	Function byte.

These together define the activity to be performed by the command message at the destination node. Note that bit five of the command byte shall always be zero (normal priority).

STS and EXT SYS – status and extended status bytes

In command messages the STS byte is set to zero. In reply messages the STS byte may contain a status code. If the four high bits of the STS byte are ones, there is extended status information in an EXT STS byte.

TNS – transaction bytes (two bytes)

The application level software must assign a unique 16 bit transaction number (generated via a counter). When the command initiator receives reply to one of its command messages, it can use the TNS value to associate the reply message with its corresponding command.

Whenever the command executor receives a command from another node, it should copy the TNS field of the command message into the same field of the corresponding reply message.

ADDR

Address field contains the address of a memory location in the command executor where the command is to begin executing. The ADDR field specifies a byte address (not a word address as in PLC programming).

Size

The size byte specifies the number of data bytes to be transferred by a message.

Data

The data field contains binary data from the application programs.

PLC-5 command set message packet fields

1	Packet offset:	This field contains the offset between the DATA field of the current message packet and the DATA field of the first packet in the transmission.
2	Total trans:	This field contains the total number of PLC-5 data elements transferred in all message packets initiated by a command.

Basic command set

The asynchronous link message packet formats to be used are delivered below:

In the lists below, privileged commands are initiated by computer and executed by PLCs. Non-privileged commands are initiated by a PLC or a computer. The CMD values listed are for non-priority command message packets.

Analog 4 to 20 mA communications

The basic communication of the HART protocol is the 4–20 mA current system. This analog system is used by the sensor to transmit an analog value to the HART PLC or HART card in a PC. In a 4–20 mA the sensor outputs a current value somewhere between 4 and 20 mA that represents the analog value of the sensor. For example, a water tank that is half full – say 3400 kilolitres – would put out 12 mA. The receiver would interpret this 12 mA as 3400 kilolitres. This communication is always point-to-point, i.e. from one device to one other. It is not possible to do multidrop communication using this method alone. If two or more devices put some current on the line at the same time, the resulting current value would not be valid for either device.

Digital multidrop communications

For multidrop communications, the HART protocol uses a digital/analog modulation technique known as frequency shift keying (FSK). This technique is based on the Bell 202 communication standard. Data transfer rate is 1200 baud with a digital ‘0’ frequency (2200 Hz) and a digital ‘1’ frequency (1200 Hz). Category 5 shielded, twisted pair wire is recommended by most manufacturers. Devices can be powered by the bus or individually. If the bus powers the devices, only 15 devices can be connected. As the average DC current of an ac frequency is zero, it is possible to place a 1200 Hz or 2200 Hz tone on top of a 4–20 mA signal. The HART protocol does this to allow simultaneous communications on a multidrop system.

The HART handheld communicator

The HART system includes a handheld control device. This device can be a second master on the system. It is used to read, write, range and calibrate devices on the bus. It can be taken into the field and used for temporary communications. The battery operated handheld has a display and key input for specific commands.

The HART field controller in Figure 11.5 is wired in series with the field device (valve positioner or other actuator). In some cases, a bypass capacitor may be required across the terminals of the valve positioner to keep the positioner’s series impedance below the 100 Ω level required by HART specifications. Communications with the field controller requires the communicating device (handheld terminal or PC) to be connected across a loop impedance of at least 230 Ω. Communications is not possible across the terminals of the valve positioner because of its low impedance (100 Ω). Instead, the communicating device must be connected across the transmitter or the current sense resistor. (Taken from the HART applications guide by the HART Communications Foundation 1999 www.hartcomm.org.)

Universal commands

• Read manufacturer and device type
• Read primary variable (PV) and units
• Read current output and per cent of range
• Read up to 4 predefined dynamic variables
• Read or write 8-character tag, 16-character descriptor, date
• Read or write 32 character message
• Read device range, units and damping time constant
• Read or write final assembly number
• Write polling address

Common practice commands

• Read selection of up to 4 dynamic variables
• Write damping time constant
• Write device range
• Calibrate (set zero, set span)
• Set fixed output current
• Perform self-test
• Perform master reset
• Trim PV zero
• Write PV units
• Trim dac zero and gain
• Write transfer function (square root/linear)
• Write sensor serial number
• Read or write dynamic variable assignments

Instrument specific commands

• Read or write low flow cut-off value
• Start, stop or clear totalizer
• Read or write density calibration factor
• Choose PV (mass flow or density)
• Read or write materials or construction information
• Trim sensor calibration

Summary of HART benefits

• Simultaneous analog and digital communications
• Allows other analog devices on the highway
• Allows multiple masters to control same smart instrument
• Multiple smart devices on the same highway
• Long distance communications over telephone lines
• Two alternative transmission modes
• Flexible messaging structure for new features
• Up to 256 process variables in any smart field device

Communication specifications

Hardware recommendations

The following formula can be used to determine the maximum cable length:

Worked example

Assume that a Model 3051C smart pressure transmitter, for a Rosemount System 3 control system, is to be installed using a shielded twisted pair. Calculate the maximum cable length permitted for reliable operation.

R = 250 ohms
C = 164 pF/m
C_f = 5000 pF

L = 1494 meters

Benefits of the modern Fieldbus

There are real benefits to be gained from the emerging networks, including:

• Greatly reduced wiring costs
• Reduced installation and startup time
• Improved on-line monitoring and diagnostics
• Easy change-out and expansion of devices
• Improved local intelligence in the devices
• Improved interoperability between manufacturers

Classes of Fieldbus networks

It would seem at first that a single Fieldbus system would be beneficial to all users, but this is not the case. Very simple field devices such as proximity switches, limit switches, and basic actuators only require a few bits of digital information to communicate an ‘off’ or ‘on’ state. These are usually associated with real time control applications where update times of a few milliseconds are required. The associated electronics necessary to communicate with these systems can be simple, compact and inexpensive enough to be integrated in the device itself.

Alternatively, complicated devices like PLCs, DCSs, or operator stations (human-machine-interfaces, HMIs), require multi-byte length messages (up to 256 in some systems) and may only require update times of 10–100 ms depending on the application. These systems require larger packets due to a large amount of data to be transferred.

The solution is to select the digital communication network that is best suited to the application, and integrate information up through the higher speed networks as required. Several approaches in digital networks have been developed over the last few years, each with a different target application, speed and technology.

These different approaches are generically referred to as Fieldbus and DeviceNet systems and are typically categorized by the length of the ‘message’ required by the devices to adequately convey information to the host or network.

This method of categorization allows these Fieldbus and DeviceNet systems to be placed in one of the following three network oriented classes:

Bit oriented systems are used, for example, with simple binary type devices such as proximity sensors, contact closures (pressure switches, float switches, etc.), simple push-button stations and pneumatic actuators. These types of networks are also known as ‘sensor bus’ networks due to the nature of the devices (sensors and actuators) typically used.

Byte oriented systems are used in much broader applications such as motor starters, bar code readers, temperature and pressure transmitters, chromatographs and variable speed drives due to their larger addressing capability and the larger information content of the several byte length message format.

Message oriented systems, which are those systems containing over 16 bytes per message, are used in interconnecting more intelligent systems such as PCs, PLCs, operator terminals and engineering workstations where uploading and downloading system or device configurations is required, or in linking the above mentioned networks together.

The OSI model and Fieldbus systems

The ISO/OSI is an internationally accepted communications reference model and as such has been universally accepted by all Fieldbus systems committees as a starting point in the design process.

As outlined, the OSI model allocates specific tasks and defines the interface for each layer. The model used in an industrial system is a simplified version with only three layers: application, data link and physical (see Figure 12.3). In addition to the three OSI model levels, a user layer is required in Fieldbus systems, to incorporate the function blocks. This is discussed later.

The functions of each layer are:

Interoperability

Interoperability is defined as the capability of using similar field devices from different manufacturers as replacements without losing functionality or sacrificing the degree of integration with the host system. The user is able to choose the right devices for an application independent of the supplier, control system and the protocol.

Refer to Figure 12.4.

The host system, from manufacturer A, can access flow meters at addresses 1, 2 and 3 from manufacturer A with full read/write capability, but only has read capability for the flow meter at address 4, from manufacturer B. Therefore, the host control system treats each of these field devices differently and they could not be used as effective replacements for each other. Only if the flow meter at address 4 is totally interchangeable with the other devices is the system considered interoperable.

Interoperability is valuable because:

• It allows the end user to select different manufacturers’ devices in an interchangeable manner with no discernible differences in the use of each device.
• The concept allows the easy integration of new field devices into control strategies, as they become available.

Importantly, a communication hierarchy, such as the OSI model, cannot address the issue of interoperability. Standardization of the physical, data link and application layers will ensure information can be exchanged among devices on a Fieldbus network. It is the User layer that actually specifies the type of data or information and how it is to be used. Hence, specification of the user layer is vital to ensure complete performance of a Fieldbus system (although it is not part of the OSI communications model).

The physical layer

AS-i uses a two-wire untwisted, unshielded cable, which serves as both communication link and power supply for up to thirty-one slaves. A single master module controls communication over the AS-i network, which can be connected in various configurations such as bus, ring, or tree (see Figure 12.5). The AS-i flat cable has a unique cross-section that permits only properly polarized connections when making field connections to the modules (see Figure 12.6). Other types of cable may be used for the AS-i network providing they meet the AS-i cable specification. A special shielded cable is also available for high noise environments.

Each slave is permitted to draw a maximum of 65 mA from the 30 V dc power supply. If devices require more than this, separate supplies must be provided for each such device. With a total 31 slaves drawing 65 mA, a total limit of 2 A has been established to prevent excessive voltage drop over the 100 m permitted network length. A 16 AWG cable is specified to ensure this condition.

The slave (or field) modules are available in four configurations:

• Input modules for 2- and 3-wire dc sensors or contact closure
• Output modules for actuators
• Input and output (I/O) modules for dual purpose applications
• Field connection modules for direct connection to AS-i compatible devices

The I/O modules are capable of accepting up to 4 I/O per slave, and a total of 124 I/O for the network.

A unique design allows the field modules to be connected directly into the bus while maintaining network integrity (see Figure 12.7). The field module is composed of an upper and lower section; secured together once the cable is inserted. Specially designed contact points pierce the self-sealing cable, providing bus access to the I/O points and/or continuation of the network. True to the modular design concept, two types of lower sections and three types of upper sections are available to permit ‘mix-and-match’ combinations to accommodate various connection schemes and device types (see Figure 12.8). Plug connectors are utilized to interface the I/O devices to the slave (or with the correct choice of modular section screw terminals) and the entire module is sealed from the environment with special seals where the cable enters the module. The seals conveniently store away within the module when not in use.

The AS-i network is capable of a transfer rate of 167 kbps. Using an access procedure known as ‘master-slave access with cyclic polling’, the master continually polls all the slave devices during a given cycle to ensure rapid update times. For example, with all 31 slaves and 124 I/O points connected, the AS-i network can ensure a 5 ms cycle time, making the AS-i network one of the fastest available.

A modulation technique called ‘alternating pulse modulation’ provides this high transfer rate capability as well as high data integrity. This technique is described in the following section.

The data link layer

The data link layer of the AS-i network consists of a master call-up and slave response. The master call-up is exactly fourteen bits in length while the slave response is 7 bits. A pause between each transmission is used for synchronization, error detection and correction. Refer to Figure 12.9 for example call-up and answer frames.

Various code combinations are possible in the information portion of the call-up frame. It is these code combinations that are used to read and write information to the slave devices. Examples of some of the master call-ups are listed in Figure 12.10. A detailed explanation of these call-ups is available from the AS-i association literature and is only included here to illustrate the basic means of information transfer on the AS-i network.

The modulation technique used by AS-i is known as ‘alternating pulse modulation’ (APM). As the information frame is of a limited size, providing conventional error checking was not possible and therefore the AS-i developers chose a different technique to insure high-level data integrity.

Referring to Figure 12.11, the coding of the information is similar to Manchester II coding, but utilizing a ‘sin squared’ waveform for each pulse. This wave-shape has several unique electrical properties, which reduce the bandwidth required of the transmission medium (permitting faster transfer rates), and reduce the end of line reflections, common in networks using square wave pulse techniques. Also, each bit has an associated pulse during the second half of the bit period. This property is used as a bit level of error checking by all AS-i devices. The similarity to Manchester II coding is due to this technique having been used for many years to pass synchronizing information to a receiver along with the actual data.

In addition, AS-i developers also established an internal set of regulations for the APM coded signal, which is used to further enhance data integrity. For example, the start bit or first bit in the AS-i telegram must be a negative impulse and the stop bit a positive impulse. Two subsequent impulses must be of opposite polarity and the pause length between two consecutive impulses should be 3 ms. Even parity and a prescribed frame length are also incorporated at the frame level. So the ‘odd’ looking wave form, combined with the rules of the frame formatting, the set of regulations of the APM coded signal, and parity checking, work together to provide timing information and a high level of data integrity for the AS-i network.

Operating characteristics

AS-i node addresses are stored in non-volatile memory and can be assigned either by the master or one of the addressing or service units. Should a node fail, AS-i has the ability to automatically reassign the replaced node’s address and in some cases reprogram the node itself allowing rapid response and repair times.

Since AS-i was designed as an interface between lower level devices, connection to higher-level systems enables the capability to transfer data and diagnostic information. Plug-in PC cards and PLC cards are currently available. The PLC cards allow direct connection with various Siemens PLCs. Serial communication converters are also available to enable AS-i connection to conventional RS-232, 422, and 485 communication links. Direct connection to a Profibus field network is also possible with the Profibus coupler, enabling several AS-i networks access to a high-level digital network.

Handheld and PC-based configuration tools are available which allow initial start-up programming and also serve as diagnostic tools after the network is commissioned. With these devices on-line monitoring is possible to aid in determining the health of the network and locating possible error sources.

The physical layer

The Seriplex cabling system consists of a single four-conductor cable with two AWG #22 shielded wires for data and clock signals and two AWG #16 wires for power and common. A shield drain wire is also provided for shield grounding. Clock rates from 16 to 100 kHz are selectable with newer versions capable of up to 200 kHz. Capacitance values of cable dramatically affect all communication systems and low capacitance cable designs are available from several manufacturers to maximize data transfer rates. Rates of up to 100 kHz over 500 feet are possible with Seriplex using low capacitance (16 pF/ft) cabling. However, 20 pF/ft cables would limit this distance at 100 kHz to 350 ft. The importance of low capacitance cabling cannot be over emphasized in any system.

12 V dc is provided by the cable to power the I/O devices in the first generation systems. Second generation systems operate on either 12 or 24 V dc, with the level selected by the user for the particular system used. Field connections are made through Seriplex modules located near the field devices.

Individual I/O addresses are programmed in the module to allow the network access to each point. A total of 255 usable addresses is available to the modules. Digital inputs and outputs use one address each. Each 8-bit analog module uses eight addresses (for one analog input or output). Multiplexing methods are employed to increase the total digital I/O count to 7,706 or analog I/O count to 480 or a combination of these.

Data and clock signals are transferred over the network in the form of 0 to +12 V digital pulses.

The data link layer

Two different methods of operation are possible with Seriplex, depending on the mode of operation. Both modes of operation use the unique access control method described in Mode 2 below.

In Mode 1, or peer-to-peer mode, modules can be logically inter-linked without the need for a host controller. In this case logical functions are implemented directly between modules. A separate clock module is required in this mode as there is no host to provide crucial clock line information. Module outputs can be logically programmed to function, based on the status of other modules’ inputs. With this capability simple logic functions can be performed without the need of a host controller.

Mode 2 operation requires the host controller to provide timing clock signals. The receiver in each module counts the clock pulses. When the pulse count of the clock line equals the receiver’s address, access is granted on the data link line for the receiver to read from and, in turn, write to, the host controller.

This access control method is unique in that a continuous ‘train’ of clock and data pulses cycle through the system. Access on the data line for individual addresses (bit status) is granted for a time period within the data stream based on the time slot of the address (see Figure 12.13). This ‘continuous polling’ starts with a synchronizing signal 8 clock cycles long and serves as notification that the ‘polling’ is about to begin. At the beginning of the cycle the data line is ‘empty’. As the pulse count equals each module’s address count, the modules ‘dump’ their bit values on the data line so that at the end of the cycle all information is available to the host. The frame size can be adjusted in length from 16 to 256 bits, in multiples of 16, to accommodate different size systems. Correct sizing of the system and resultant frame size can provide extremely fast update times for smaller networks.

Data echoing provides error detection on the bit level. The receiver echoes messages (which are typically one bit per address) to confirm correct data receipt. This is not automatic and is implemented in the application layer of the software by the applications programmer.

Operating characteristics

Seriplex is a bit oriented network system intended to link lower level devices both physically and, in Mode 1 operation, logically. These features are incorporated in the AS-I-C chip located in all Seriplex devices. Handheld programming devices are available to enable Seriplex device configuration.

Interface devices to higher-level field networks through host controllers or special Gateways are also available.

CANbus

The CAN network was developed in the automotive industry in response to the rapidly growing use of electronic-control systems in automobiles. As demands for fuel efficiency and safety increased, more and more electronic devices became part of the system. The need for multiple devices to pass information between them rapidly became a necessity. A type of serial data bus system was developed by Bosch to meet these demands. It was called the controller area network or CAN. CAN is formally specified in: 1) BOSCH CAN specification – Version 2.0, Part A, and 2) ISO 11898: 1993 – road vehicles –Interchange of digital information – controller area network (CAN) for high-speed communication. CAN has since been rapidly adapted to industrial applications.

CAN is a bus type network system which does not use a bus master or token passing schemes to access the bus. Instead it uses a unique access control method called ‘non-destructive bit-wise arbitration’. This type of access control uses the station identifier bit pattern itself to gain access to the bus as shown in Figure 12.14. The priority of the station is determined by the addressing assignments during configuration of the network and allows the station with the highest priority preferred access. Unlike token passing or master-slave type arbitration schemes CAN is not deterministic, but defers to the station with highest priority making the lower priority stations wait for access.

Figure 12.14 shows the bit arbitration of a CAN type system. The devices 1, 2, and 3 try to transmit at the same time. Ground or a ‘0’ is dominant. The results can be seen on the top waveform. Because device 1 puts out a ‘1’ and it is dominated by the ‘0’ from 2 and 3, it loses and stops transmitting. Then device 3 puts out a ‘1’ and is dominated by 2. Therefore, 2 continues to transmit while 1 and 3 wait until the line is clear.

The CAN station that wins the arbitration continues to transmit its message frame uninterrupted with no corruption from the other stations arbitration attempts. This allows higher efficiency of data transfer over the network. A typical CAN message frame is shown in Figure 12.15. Notice the data field can be of variable length – up to 8 bytes. This makes CAN suitable for use with more sophisticated devices that may require several bytes to adequately convey their information content. CRC error checking and specific frame length requirements, as well as individual message acknowledgments, are used to ensure data integrity over the bus.

The CAN protocol specifications cover only the physical (layer 1) and data link (layer 2) layers of the ISO/OSI model. Specifics concerning the physical medium for the communication link and the application layer (layer 7) are left to the designers of the systems as described below.

DeviceNet

DeviceNet, developed by Allen-Bradley, is a low-level device oriented network based on the CAN network. It is designed to interconnect lower level devices (sensors and actuators) with higher-level devices (controllers). The variable, multi-byte format of the CAN message frame is well suited to this task as more information can be communicated per message than with the bit type systems.

The Open DeviceNet Vendor Association Inc. (ODVA) has been formed to issue DeviceNet specifications, ensure compliance with the specifications and offer technical assistance for manufacturers wishing to implement DeviceNet. Over 125 firms have either joined formally or have signed intent to become members. The DeviceNet specification is an open specification and available through the ODVA.

DeviceNet can support up to 64 nodes, supporting as many as 2048 total devices. A single, four-conductor cable provides both power and data communications. Various devices are available to interconnect I/O devices, and the network trunk-line cable allowing customized configurations.

As DeviceNet was designed to interface lower level devices with higher-level controllers, a unique adaptation of the basic CAN protocol was developed. This is similar to the familiar poll/response or master/slave technique, but still utilizes the speed benefits of the original CAN.

Figure 12.16 shows the DeviceNet profile in its relationship to the ISO/OSI Model. It is important to note that only Layers 1 and 2 are covered by the CAN protocol specification while the remaining layers were developed for the DeviceNet network.

The following sections examine these features of the DeviceNet network and protocol in more detail.

The physical layer

The DeviceNet cabling system consists of a single four-conductor cable in a bus topology providing both power and data communication. Data is transmitted over the #18 twisted pair. Power is provided over the #15 twisted pair. Both pairs have a foil shield and an overall braid with #18 drain wire. Terminating resistors are required on both ends of the trunk line. 24 V dc power is provided on the power bus and can support up to 3 Amp on the DeviceNet thin cable or 8 amp on the DeviceNet thick cable. The total length of trunk line allowed depends on which type of cable is used, the number of devices supported and the data rate. The voltage at each device should be at least 11 V dc or higher.

Data rates of 125, 250, and 500 kbps are possible with the corresponding network configuration shown in Figure 12.17. Various connectors can be used to connect devices to the network such as screw terminals, screw terminals or sealed screw tight connectors.

To allow non-destructive arbitration during simultaneous transmission from two or more nodes the BOSCH CAN specification defines the two possible logic levels as ‘dominant’ and ‘recessive’. During arbitration the dominant value will win access to the bus. For DeviceNet, the dominant level is represented by a logical ‘0’ and the recessive level by a logical ‘1’. The electrical voltage levels representing these logic levels are taken from the ISO 11898 standard. CAN utilizes a balanced transmission system with data signals appearing as the difference between CAN_H and CAN_L.

DeviceNet specifications require isolation to prevent ground loops. As the circuitries in all devices are ultimately referenced to the V-bus signal, connection of the network should be earth grounded at the bus power supply only. All devices attached to the network must either be referenced to V– or otherwise ground isolated.

DeviceNet requires the following features to be incorporated within the physical and media layers:

• Use of CAN technology
• Support of both thick and thin drop line
• Ability to operate at a minimum of three data rates
• 125 kbaud for distances up to 500 meters (max.) (1640 feet)
• 250 kbaud for distances up to 200 meters (max.) (656 feet)
• 500 kbaud for distances up to 100 meters (max.) (328 feet)
• Linear bus topology
• Low loss and low delay cable
• Shielded twisted pair cable, containing both power and signal pairs
• Small size and low cost
• Support of up to 64 nodes
• Support drop lines of up to 6 meters (20 feet) in length
• Node removal without disturbing or interrupting network operation
• Simultaneous support of isolated and non-isolated physical layers

DeviceNet uses two types of pre-made cables, thick cable and thin cable. The thick cable is a large gray cable used as long trunk runs between devices. The thin cable is a small and usually short yellow cable that connects thick cables to devices. The thick and thin cables are connected together by a large black ‘T’ junction. All cables and connections have threaded ring connectors.

Smart distributed system (SDS)

The smart distributed system (SDS) was developed by Honeywell and is a low-level device oriented network based on the CAN network. It is designed to interconnect lower level devices (sensors and actuators) with higher-level devices (controllers). The variable, multi-byte format of the CAN message frame is well suited to this task since more intelligence can be communicated per message than with the bit type systems.

The SDS ‘partners’ program has been formed, and in cooperation with Honeywell issues the SDS specifications, ensures compliance and offers technical assistance for manufacturers wishing to implement SDS. The SDS specification is an open specification and available through Honeywell and the SDS ‘partners’ program.

The SDS network can connect up 126 devices on a single bus. Each group of 16 I/O is interfaced to a higher-level device (PLC, for example) through the interface terminal strip (ITS). The ITS provides the physical interface between the network bus and individual I/O points on the PLC I/O cards. Plug-in cards are also available to interface the bus directly to the PC. This choice between interfaces gives the designer a method for integrating the SDS with an existing PLC system.

The SDS utilizes the CAN network to allow devices to report information only when there is a need, e.g. a change of state of an input to the controller. This approach reduces traffic on the network by minimizing polling inquires from the controller to the slave devices.

As with other CAN based systems the SDS network uses the OSI Layers 1 and 2 (physical and data link layers) of the CAN protocol and develops the SDS application layer (OSI Layer 7) for its specific target application area, integrating lower level devices with higher level controllers.

The following sections examine these features of the smart distributed system (SDS) network and protocol in more detail.

The physical layer

The Interbus-S cabling system specification allows for either twisted pair copper or fiber optic cable connected to each station in a ring topology. Communication is serial and frame transmission is accomplished through a unique register shifting procedure developed specifically for the Interbus-S network. Two types of communication buses are used as part of the same network – local bus and remote bus. Each bus type carries the same signals but at different electrical levels. Local bus operates at TTL voltage levels and is designed for short distances typically within a control enclosure. Remote bus utilizes RS-485 voltage levels and is designed to communicate over much longer distances – up to 1300 ft (400 m). Both buses operate at 500 kbps transfer speed and a special module; the BK module is required to translate the two signal levels (Figure 12.18).

Communication is performed through scan cycles. Each scan cycle shifts messages through each station in increasing order through the network. Data is read from the message and written to the message during each cycle making for very fast response times (see Figure 12.19).

The data link layer

The data link layer protocol provides for full duplex transmission. The complete message frame is clocked through the network on each cycle. No arbitration or contention for access to the network is required since each station has access during each cycle. All input and output data are updated and transferred during each scan cycle. CRC error checking is performed between each network connection allowing identification of the error source. Both digital and analog data are supported as well as client/server messaging.

The network does individual station addressing automatically during initialization of the network, eliminating the need to manually assign these during system startup. This is accomplished through an identification (ID) cycle that tells the controller the type and physical order of location of the stations on the network (see Figure 12.20).

The application layer

Interbus-S supports network and module diagnostics and monitoring through a special ‘telegram’ message sent out after each byte is shifted. All stations monitor this message simultaneously. This unique function is accomplished by a ‘telegram control’ switch in each station (see Figure 12.21) which automatically activates after each 8-bit shift allowing not only simultaneous reception of the message by all stations, but synchronization information as well.

Figure 12.22 shows the ID and scan transmission frames noting the location of the various field parameters.

Introduction to Profibus

Profibus is an open standard Fieldbus defined by the German DIN 19245 Parts 1 and 2. It is based on a token bus/floating master system. There are three different types of Profibus – FMS, DP and PA. Fieldbus message specification (FMS) is used for general data acquisition systems. DP is used when fast communications are needed. And PA is used in areas when intrinsically safe devices and intrinsically safe communications are needed. Figure 12.23 outlines the structure of the various versions of Profibus.

The physical layer

The physical layer specifies the type of Profibus transmission medium. The RS-485 voltage standard is defined for the FMS and DP versions of Profibus. The IEC 1158-2 standard is used in the PA version. For FMS and DP a maximum number of 255 stations are possible.

Basic properties of the RS-485 voltage standard for Profibus

IEC 1158-2 is a standardized current standard used in special areas of a factory or plant that require intrinsically safe devices. IEC 1158-2 works by modulating a Manchester encoded bipolar NRZ ±10 mA signal on top of a 9 to 32 dc voltage. This 10 mA creates a ±1 volt signal that is read by each of the devices on the bus.

It is very easy to connect Profibus FMS, DP and PA versions together on the same system, as the main difference between the FMS, DP and PA versions is the physical layer. This allows a company to run lower cost devices in most of the plant (FMS), fast devices (DP) in parts of the plant that need the speed. Intrinsically safe devices (PA) are used in the areas of the plant that need intrinsically safe devices.

The data link layer

The data link layer is defined by Profibus as the Fieldbus data link Layer (FDL).

The medium access control (MAC) part of the FDL defines when a station may transmit data. The MAC ensures that only one station transmits data at any given time.

Profibus communication is termed hybrid medium access. It uses two methods of operation:

• Token passing
• Master/slave.

By means of a software token, the token passing method ensures (which when passed from node to node assigns the right of transmission to that node), the assignment of the bus access right within a precisely defined time interval. This is circulated with a maximum (and configurable) token rotation time between all masters. Token passing is especially useful for communication between complex automation masters who require equal rights on the bus. The token is passed in a defined sequence (in order of increasing addresses).

The master/slave method allows the master that currently has the token to communicate with the associated slave devices. The master can then read from or write data to the slave devices.

A typical configuration is shown in Figure 12.24.

During the Bus system startup phase, the task of the active station’s MAC is to detect logical assignment and establish the token ring. The MAC also handles adding or deleting stations (which have become inactive), deleting multiple nodes with the same address and multiple or lost tokens.

The application layer

This consists of two sections:

• Fieldbus message specification (FMS)
• Lower layer interface (LLI)

The application layer is defined in DIN 19245 part 2.

The Profibus communication model

The part of the application process in a field device that is readable for communication is called the virtual field device (VFD). The VFD contains the communication objects that may be manipulated by the services of the application layers. The objects of a real device that are readable for the communication (variables, programs, data domains) are called communication objects.

All communication objects of a Profibus station are entered into its local object dictionary (source OD). There are two types:

• Static communication objects
• Dynamic communication objects

Static communication objects are defined in the static object dictionary. They may be predefined by the manufacturer of the device, or defined during the configuration of the bus system. Static communication objects are used mainly for communication in the field area. Profibus recognizes the following static communication objects:

• Simple variable
• Array – sequence of simple variables of the same type
• Record – sequence of simple variables, not necessarily of the same type
• Domain – data range
• Event

Dynamic communication objects are entered into the dynamic part of the OD (list of variable lists of program invocations). They may be predefined or defined, deleted or changed by the application services in the operational phase.

Profibus supports the following dynamic communication objects:

• Program invocation
• Variable list (sequence of simple variables, arrays or records).

There are two methods for accessing the variables:

• Addressing by name (using a symbolic name)
• Physical addressing (to access a physical location in memory)

Profibus defines logical addressing (by symbolic name) as the preferred method as this increases the speed of access.

Application services

From the point of view of an application process, the communication system is a service provider offering various application services – the FMS services. The FMS describes the communication objects, the application services, and the resulting models from the viewpoint of the communication partner. There are two types of services:

Note:

Refer to ‘Connectionless-oriented’ and ‘Connection-oriented’ under Lower layer interface following for an explanation of these terms.

Service primitives in the Profibus standard describe the execution of the services. The services can be divided into the following groups:

• Context management services allow establishment and release of logical connections
• Variable access services permit access to simple variables, records, arrays and variable lists
• Domain management services enable the transmission of contiguous memory areas
• Program invocation management services allow the control of program execution
• Event management services make the transmission of alarm messages possible
• VFD support services permit device identification and status report
• OD management services permit object dictionaries to be read and written

Lower layer interface (LLI)

The LLI conducts the data flow control and connection monitoring as well as the mapping of the FMS services onto the layer 2 with consideration of the various types of devices.

The user communicates with other application processes over the logic channels, the communication relationships. For the execution of the FMS and FMA7 services the LLI provides various types of communication relationships.

There are two types of communication relationships:

• Connection-oriented relationships
  This requires a connection establishment phase (or initiate service) before the connection can be used for data transmission. When the connection is no longer required, it may be released with the abort service (or connection release phase). The connection attribute distinguishes between defined connections where the communication partner is fixed at a configuration time and may not be changed and an open connection where the communication partner is dynamically defined in the connection establishment phase.
• Connectionless-oriented relationships
  Cyclic data transfer means exactly one variable is permanently read or written over a connection. A typical application for cyclic data transfer is the periodic update of the remove inputs and outputs of a PLC. Acyclic data transfer means an application sporadically accesses various communication objects over a connection.

Communication relationship list (CRL)

The CRL contains the description of all communication relationships of a device independent of the time of their usage.

Bus access method

The broadcasting approach is used, with a central unit (called the bus arbitrator) co-coordinating the transmissions. This means that it is not necessary to give each device a unique address. A variable (processed by the transmitter only) is transmitted on the bus by one transmitter and is read by any number of receivers situated on the same bus.

The bus arbitrator has three operating cycles:

• Cyclic traffic
  The bus arbitrator names a set of variables using a table command.
• Aperiodic traffic
  The bus arbitrator calls on request variable from every device.
• Message service
  The arbitrator gives the right to transmit to a device, which requested it during the previous cyclic traffic period.

The physical layer

The FIP standard allows twisted pairs, optical fiber or coaxial cable in a bus topology to a maximum distance of 2 km. Speeds can vary between 31.25 and 1000 kbps. A maximum number of 256 devices is allowed on the bus network.

The data link layer

The data link layer is non-proprietary FIP.

Installations

There are a number of installations in France and Italy that are being used to evaluate the FIP standard.

The FIP standard has evolved into the WorldFIP standard as discussed in the next section.

Introduction to Foundation Fieldbus

Prior to 1994, two organizations were independently trying to develop a communication solution standard for the process control industry. These organizations were WorldFip (North America) and Interoperable Systems Project (ISP). In September 1994 these two organizations merged to become the Foundation Fieldbus (FF).

Foundation Fieldbus takes full advantage of the emerging ‘smart’ field devices and modern digital communications technology allowing end user benefits such as:

• Reduced wiring
• Communications of multiple process variables from a single instrument
• Advanced diagnostics
• Interoperability between devices of different manufacturers
• Enhanced field level control
• Reduced startup time
• Simpler integration

The concept behind Foundation Fieldbus is to preserve the desirable features of the present 4–20 mA standard (such as a standardized interface to the communications link, bus power derived from the link and intrinsic safety options) while taking advantage of the new digital technologies. This will provide the features noted above due to the:

• Reduced wiring due to the multidrop capability
• Flexibility of supplier choices due to interoperability
• Reduced control room equipment due to distribution of control functions to the device level
• Increased data integrity and reliability due to the application of digital communications.

To understand how this standard works, it is helpful to look at Foundation Fieldbus in terms of the OSI Model. The FF consists of three parts that correspond to OSI layers 1, 2, 7 and 8. Layer 8 of the OSI model corresponds to the ‘user’ layer.

The physical layer and wiring rules

The physical layer standard has been approved and is detailed in the IEC 1158-2 and the ISA standard S50.02-1992. It supports communication rates of 31.25 kbps and 10 Mbps. All of these use the Manchester bi-phase L encoding scheme with four encoding states as shown in Figure 12.28. The use of the N+ and N– encoding states is illustrated in Figure 12.29. Devices can be optionally powered from the bus under certain conditions as detailed below for the various configurations. The 31.25 kbps (or H1, or low-speed bus) can support from 2 to 32 devices that are not bus powered, two to twelve devices that are bus powered or two to six devices that are bus powered in an intrinsically safe area. Repeaters are allowed and will increase the length and number of devices that can be put on the bus. The H2 or high speed bus options are not currently being implemented, but have been superseded by the high speed Ethernet (HSE) standard. This is discussed later in this section.

The low speed bus was intended to utilize existing plant wiring and is referred to as Type B wiring (shielded twisted pair) and with #22 AWG can be used for segments up to 1200 m (3936 feet). The higher speeds require higher grade cabling and are referred to as Type A. For Type A cable (shielded twisted pair) for H1 #18 AWG can be used up to

1900 meters (6232 feet). Two additional types of cabling are specified and are referred to as Type C (multi-pair twisted without shield) and Type D (called multi-core, no shield). Type C using #26 AWG cable is limited to 400 meters (1312 feet) per segment and Type D with #16 AWG is restricted to segments less than 200 meters (660 feet).

The Foundation Fieldbus wiring method is floating balanced and equipped with a termination resistor combination connected across each end of the transmission line. Neither of the wires should ever be connected to ground. The terminator consists of a 100 Ω quarter watt resistor and a capacitor sized to pass 31.25 kHz. As an option one of the terminators can be center tapped and grounded to prevent voltage build-up on the Fieldbus. Power supplies must be impedance matched for FF. Off the shelf power supplies must be conditioned. If a ‘normal power supply’ is placed across the line it will load down the line due to its low impedance. This will cause the transmitters to stop transmitting.

Fast response times for the bus are one of the FF goals. For example, at 31.25 kbps on the H1 bus response times as low as 32 microseconds are expected (this will vary based on the loading of the system but will average between 32 ms and 2.2 ms with an average approximately 1 ms).

Spurs can be connected to the ‘home run’. The length of the spurs depends on the type of wire used and the number of spurs connected. The maximum length is the total length of the spurs and the home run.

The physical layer standard has been out for some time. Most of the recent work has been focused on these upper layers and are defined by the FF as the ‘communications stack’ and the ‘user layer’. The following sections will explore these upper layers. Figure 12.30 helps understanding the subsequent discussions.

The data link layer

The communications stack as defined by the FF corresponds to OSI layers two and seven, the data link and applications layers. The DLL (data link layer) controls access to the bus through a centralized bus scheduler called the link active scheduler (LAS). The DLL packet format is shown in Figure 12.31.

The link active scheduler (LAS) controls access to the bus by granting permission to each device according to predefined ‘schedules’. No device may access the bus without LAS permission. There are two types of schedules implemented: cyclic (scheduled) and acyclic (unscheduled). It may seem odd that one could have an unscheduled ‘schedule’, but these terms actually refer to messages that have a periodic or non-periodic routine, or ‘schedule’.

The cyclic messages are used for information (process and control variables) that require regular, periodic updating between devices on the bus. The technique used for information transfer on the bus is known as the publisher–subscriber method. Based on the user predefined (programmed) schedule the LAS grant permission for each device in turn access to the bus. Once the device receives permission to access the bus it ‘publishes’ its available information. All other devices can then listen to the ‘published’ information and read it into memory (subscribe) if it requires it for its own use. Devices not requiring specific data simply ignore the ‘published’ information.

The acyclic messages are used for special cases that may not occur on a regular basis. These may be alarm acknowledgment or special commands such as retrieving diagnostic information from a specific device on the bus. The LAS detects time slots available between cyclic messages and uses these to send the acyclic messages.

The application layer

The application layer in the FF specification is divided into two sub-layers – the Foundation Fieldbus access sublayer (FAS) and the Foundation Fieldbus messaging specification (FMS).

The capability to pre-program the ‘schedule’ in the LAS provides a powerful configuration tool for the end user since the time of rotation between devices can be established and critical devices can be ‘scheduled’ more frequently to provide a form of prioritization of specific I/O points. This is the responsibility and capability of the FAS. Programming the schedule via the FAS allows the option of implementing (actually, simulating) various ‘services’ between the LAS and the devices on the bus.

Three such ‘services’ are readily apparent such as:

These variations, of course, depend on the actual application and one scheme would not necessarily be ‘right’ for all applications, but the flexibility of the Foundation Fieldbus is easily understood from this example.

The second sub-layer, the Foundation Fieldbus messaging specification (FMS), contains an ‘object dictionary’ which is a type of database that allows access to Foundation Fieldbus data by tag name or an index number. The object dictionary contains complete listings of all data types, data type descriptions, and communication objects used by the application. The services allow the object dictionary (application database) to be accessed and manipulated. Information can be read from or written to the object dictionary allowing manipulation of the application and the services provided.

The user layer

The FF specifies an eighth layer called the user layer that resides ‘above’ the application layer of the OSI model; this layer is usually referred to as Layer 8. In the Foundation Fieldbus this layer is responsible for three main tasks – network management, system management and function block/device description services. Figure 12.32 illustrates how all the layer’s information packets are passed to the physical layer.

The network management service provides access to the other layers for performance monitoring and managing communications between the layers and between remote objects (objects on the bus). The system management takes care of device address assignment, application clock synchronization, and function block scheduling. This is essentially the time coordination between devices and the software, and ensures correct time stamping of events throughout the bus.

Function blocks and device description services provide pre-programmed ‘blocks’, which can be used by the end user to eliminate redundant and time-consuming configuration. The block concept allows selection of generic functions, algorithms, and even generic devices from a library of objects during system configuration and programming. This process can dramatically reduce configuration time since large ‘blocks’ are already configured and simply need to be selected. The goal is to provide an open system that supports interoperability and a device description language (DDL), which will enable multiple vendors, and devices to be described as ‘blocks’ or ‘symbols’. The user would select generic devices then refine this selection by selecting a DDL object to specify a specific vendor’s product. Entering a control loop ‘block’ with the appropriate parameters would nearly complete the initial configuration for the loop. Advanced control functions and mathematics ‘blocks’ are also available for more advanced control applications.

Error detection and diagnostics

FF has been developed as a purely digital communications bus for the process industry and incorporates error detection and diagnostic information. It uses multiple vendors’ components and has extensive diagnostics across the stack from the physical link up through the network and system management layers by design.

The signal method used by the physical layer timing and synchronization is monitored constantly as part of the communications. Repeated messages and the reason for the repeat can be logged and displayed for interpretation.

In the upper layer, network and system management is an integral feature of the diagnostic routines. This allows the system manager to analyze the network ‘on-line’ and maintain traffic loading information. As devices are added and removed, optimization of the link active scheduler (LAS) routine allows communications optimization dynamically without requiring a complete network shutdown. This ensures optimal timing and device reporting, giving more time to higher priority devices and removing, or minimizing, redundant or low priority messaging.

With the device description (DD) library for each device stored in the host controller (a requirement for true interoperability between vendors) all the diagnostic capability of each vendors’ produces can be accurately reported and logged and/or alarmed to provide continuous monitoring of each device.

High-speed Ethernet (HSE)

High-speed Ethernet (HSE) is the Fieldbus Foundation’s backbone network running at 100 Mbps. HSE Field Devices are connected to the backbone via HSE linking devices. A HSE linking device is a device used to interconnect H1 Fieldbus segments to HSE to create a larger network. A HSE switch is an Ethernet device used to interconnect multiple HSE devices such as HSE linking devices and HSE field devices to form an even larger HSE network. HSE hosts are used to configure and monitor the linking devices and H1 devices. Each H1 segment has its own link active scheduler (LAS) located in a linking device. This feature enables the H1 segments to continue operating even if the hosts are disconnected from the HSE backbone. Multiple H1 (31.25 kbps) Fieldbus segments can be connected to the HSE backbone via linking devices.

Star topology

In the star configuration, there is a central node or hub and all the outlying nodes communicate back to it on separate communication links. A typical example of a star network is a timesharing mainframe computer system, where the central node is the computer itself and the outlying nodes are the user terminals. With the star network, each outlying node is connected to the central node via its own cable. Figure 13.1 shows the topology of a star network.

The central hub must have the capacity to simultaneously send and receive messages. Contention problems are overcome by using buffers in the computer at the hub of the star network.

The major disadvantages of the star network are:

• If the central hub is disabled, the entire system is inoperable.
• Depending on the physical layout, the costs of cabling a star network tend to be higher than some of the alternatives.

Ring topology

Nodes in a ring network are connected node to node, ultimately forming a loop. The data flow is often arranged to be unidirectional, with each node passing data on to the next node and so on. It is essential that each node, when receiving, is capable of removing data from the ring so that it does not circle through the network indefinitely.

The basic ring network is unable to function if a node is disabled or the ring broken. Consequently, if modifications or additions are to be made to the network, a complete system shutdown is necessary. Figure 13.2 shows the topology of a ring network.

Bus topology

The bus topology consists of a communication path with nodes connected to it like leaves off a branch. The nodes are not physically inserted into the bus, as is the case with ring topology, but are ‘teed-off’ the bus.

Bus networks can be bi-directional or unidirectional. Figure 13.3 shows the topology of some typical bus networks.

Data on the bus can be ‘seen’ by all the nodes, but only the destination node or nodes will copy the information.

In the event of a disabled node, either due to a failure or an access control malfunction (i.e. the destination node not taking its information from the bus), the network can continue to operate as before, without the malfunctioning node.

As any of the devices on the network can send data at any time, there is a potential ‘traffic control’ problem. A number of protocols have been developed to regulate the access of devices to the bus network to avoid data collisions. Ethernet is one of the best known and most successful of the LANs using the bus topology. It is described in more detail in the following sections.

Master-slave (or poll-response) mode

This is a common method of gathering data from an instrument connected to a bus. When one master node is connected to several slave nodes, the master-slave method works as follows:

• The master node sends a message addressed to the first slave, in a sequence of nodes, requesting data (or writing data to the slave).
• All the slave nodes on the bus read the message, but if the destination address of the message does not match their own address, they discard the message.
• The correctly addressed slave node reads the message and checks for any errors (e.g. frame size and CRC or BCC errors).
• If the addressed slave node does not immediately respond, the master node typically attempts to transmit three read requests (in total) before retrying to transmit a message to the next slave node in the sequence.
• The master node then cycles through all the nodes on the bus requesting data (or writing data) to each of them. A complete cycle is referred to as a polling-cycle.

The master node may need to retry a failed node every few polling-cycles to see if it has recovered and now wants to transfer data.

Depending on the configuration, the physical link can be used in a half-duplex or full-duplex mode. For example, using RS-232 would allow full-duplex mode for only two nodes, (one at each end of the link).

The advantage of this approach is that the software is easy to setup as the master node is in total control and will never receive unsolicited messages. The polling-cycle approach is very efficient if any one, or all, of the following situations exist:

• Each slave has a constant predictable amount of data to be read and written to it.
• There is no need to transfer data between slave nodes (but only from the slave node back to the master node).
• A slave node will never need to jump ahead of the polling queue to transfer urgent data back to the master node.
• There is a very low probability of failure of a slave or master node.

The disadvantage of this approach is that when a slave node has an unpredictable amount of data and may want to get onto the bus at certain times to transfer critical data to the master or any other node.

The polling-cycle approach is commonly used for instruments connected back to a central node (such as a PLC or computer). The physical topology used is typically a bus type network.

Token passing

Token passing is a common methodology for industrial control systems requiring a guaranteed transfer of data between nodes that are network peers.

There are two types of messages:

• Token messages (which transfer control of the network from one node to the next)
• Application messages (which transfer actual data from one node to the other)

The approach would work as follows for a group of nodes connected on a bus network:

• A node receives the token message from a neighbor node, indicating that it now has control.
• The token will remain at this node for a specified maximum length of time or until it has transferred it messages, whichever is the shorter period of time.
• This node now transmits messages to other nodes with which it wants to communicate.
• The token is forwarded to the next node and the process repeats itself.

The use of a token ensures that each node on the network will be allowed to transmit within a given time slot. This is called deterministic operation.

Tokens can be used on any network topology. Examples are:

• Arcnet (star)
• Modbus (bus)
• IBM token ring (ring)

This is very common method where transfer of data (such as alarm conditions) is vital and worst-case access time to the medium must be guaranteed.

Carrier sense multiple access/collision detection (CSMA/CD)

CSMA/CD is the simplest method of passing data, on a bus, between nodes that want to communicate in a peer-to-peer fashion. It is becoming increasingly popular in industrial systems because of its ease of implementation and low cost.

The system works as follows for nodes connected to a bus:

• A node that wants to transmit first listens for any bus activity. If there is no activity detected the node will transmit a message.
• As the node transmits a message it compares the message being sent with that being present on the bus. If it detects a mismatch it will immediately stop transmitting as this signifies an error on the system (either due to noise or to another node or nodes transmitting at the same time).
• If there is a collision, the affected node will stop transmitting and wait for a random period of time before trying to transmit again.
• The node backs off for a random time. This reduces the risk of a collision when the node tries to retransmit.

The advantages of CSMA/CD are its simplicity and speed, when the bus is lightly loaded.

The disadvantage with CSMA/CD is that all activity on the bus is unpredictable (and the greater the loading of the bus the more unpredictable it becomes). In fact, during extreme high-activity periods the bus may almost cease to function and no data transferred.

CSMA/CD uses a bus topology and operates over a half-duplex connection. The most common example of CSMA/CD is Ethernet (or the IEEE 802.3). Canbus (or DeviceNet and SDS) use a similar version called CSMA/CA carrier sense multiple access/collision avoidance, as discussed in section 12.5. The difference here is that collisions are not destructive and the highest priority node continues transmitting and all others stop.

Baseband

This is also known as time division multiplexing (TDM). Only one device is allowed to transmit at any one time and can use the entire bandwidth of the system. No carrier is used, so the signal (e.g. the output from a UART) is directly applied to the medium.

As one communicating device takes over the medium during transmission, there has to be a time limitation on individual device access so that other devices can take their turns.

Broadband

Broadband is also known as frequency division multiplexing (FDM). The system bandwidth is divided into channels that do not overlap, meaning that many pairs of devices can communicate simultaneously and they usually retain their channel until the message transfer is complete. As only a part of the system bandwidth is available, data transfer rates for individual communications are less than for TDM using the same physical setup.

Data is transmitted by injecting a carrier (sine) wave on to the medium and modulating the carrier with the data – be that by frequency modulation, amplitude modulation, or phase modulation.

Coaxial cable and optical fiber are preferred for FDM because the bandwidth of a twisted pair is generally not sufficient to make the technique worthwhile. However, special new twisted pair types such as TwistLAN offer up to 16 Mbps, making them also suitable for FDM use.

Interpreting message packets

Generating appropriate responses to errors and acknowledgment of message packets

Both Ethernet and IBM’s token ring are compatible with this section.

• IEEE 802.3 (ISO 8802.3)
  This standard defines the carrier sense multiple accessing with collision detection (CSMA/CD) protocol, which is the protocol used by Ethernet and is described in the section on Ethernet.
• IEEE 802.4 (ISO 8802.4)
  This standard defines the token passing bus access method. Physically this looks like a bus network with the operation like that of a token ring network. Nodes on the network see themselves as being arranged in a logical loop, with each node assigned an address. Collision problems that occurred in CSMA/CD are solved as only one token can exist on the network at one time, and only one node may own the token. Token holding times exist so that no node may own the period for prolonged periods of time. Token bus is superior to CSMA/CD for networks with heavy loads, because each node has a regular time to communicate.
• IEEE 802.5 (ISO 8802.5)
  This standard defines a token ring access method, similar to that used by IBM for their token ring standard.

Ethernet topology

Standard and thin Ethernet use a bus topology, in which each node attaches to the communications cable as shown in Figure 13.4. In this arrangement the removal of a node does not affect operation of the LAN; it simply means that the node will then not reply to frames addressed to it.

10BaseT (Ethernet) uses a star configuration in which each node is connected via two twisted pairs to a wiring hub as shown in Figure 13.5. A hub may be connected to some other type of Ethernet; operation is the same as for the bus configuration.

Data frame

All data transfer is in the form of a packet or frame. It consists of an envelope containing control information (such as synchronization bytes and addresses) and the actual message data. Each node examines the destination address and reads the data if the frame is directed at that node.

An Ethernet data frame has the structure shown in Figure 13.7.

Data encoding and transmission

Clock signals are encoded into the transmitted bit stream and extracted by each receiver for synchronizing purposes. That is, the system is ‘self-clocking’.The encoding and decoding process is illustrated in Figure 13.8. The original data stream varies between two levels and does not return to zero symbols. This is known as a non-return to zero (NRZ) signal. This data is encoded for transmission using Manchester (or phase) encoding where a binary 1 is encoded as a ‘low to high’ transition in the middle of each bit and a binary 0 as a ‘high to low’ transition. There is always a transition in the middle of each bit and this is used to extract the received clock signal. This received clock signal defines when the receiver samples the encoded data, which is either high (for binary 1) or low (for binary 0), which ensures a correct reproduction of the original data.

Standard (Thick) Ethernet

The standard 10Base5 Ethernet requires a 10.28 mm outside diameter 50 ohm cable (RG-8) that can carry a clamp-on tap forming a T junction. A transceiver media attachment unit (MAU) is joined to the tap. A transceiver drop cable or attachment unit interface (AUI) connects the MAU to the media access control (MAC) unit. In the case of an instrumentation system, the MAC unit is built into the controller card and includes the protocol control firmware. This makes up the communication subsystem, acting as a DTE.

Within the transceiver, the jabber control section disconnects the node from the bus if it detects excessive transmission activity. This prevents a faulty node from disrupting all network communications. The collision detection section implements the CSMS/CD method described previously.

MAU

The MAU can be attached to the cable in one of two ways:

• A ‘vampire’ tap. In older systems it was necessary to predrill a hole for connection to the center conductor of the coaxial cable. Modern systems use a simple insulation displacement method whereby ‘spikes’ are driven into the screen and center conductor by means of a small wrench or screwdriver.
• An N-connector MAU has two female N-connectors. The cable has to be cut and fitted with two female N connectors. This approach is preferable in a dirty environment such as a factory.

An MAU can have one, two or four ports. The standard requires a minimum interval of 2.5 meters (8.2 feet) between MAUs. A 500 meter (1640 feet) segment (the maximum) can have up to 100 MAUs connected.

AUI

The AUI is a 15-conductor shielded cable, consisting of five individually shielded pairs and may be up to 50 meters (164 feet) long. Figure 13.10 illustrates an AUI.

It is recommended that all cabling within a system be taken from the same reel to avoid reflections due to impedance mismatches. Splicing is achieved via male N-connectors on the cable ends, joined by a female-female connector barrel.

Thin Ethernet

A thin Ethernet system uses 50 Ω RG-58 A/U or C/U cable and can have a maximum segment length of 185 m (607 feet). It is designed for lower installation costs.

The setup is the same as for standard Ethernet, except that the minimum spacing between MAUs is 0.5 m (1.7 feet) and up to 30 MAUs are allowed per segment. A segment cable should not be spliced, but joined with barrel connectors.

At a controller, the MAU and AUI are typically integrated into the card as shown in Figure 13.12.

10Base5 components can be used on 10Base7, but the MAU cannot use a vampire tap or N-connector. The cable is different therefore a BNC T-piece is used.

10BaseT

A 10BaseT system provides lower installation costs again and uses twisted pair cables. Each node is connected to a central hub by two pairs (one for receiving and one for transmitting). The hub can be up to 100 m (330 feet) away. Connection to the interface cards is by modular RJ-45 8-pin plugs as shown in Figure 13.13.

A hub provides a bus between the nodes and may incorporate AUI connectors for thick Ethernet, thin Ethernet or fiber optic transceivers.

Topology

This is a ring network in which the transmission medium forms a closed loop. Data is transmitted in one direction as shown in Figure 13.14.

Note that failure at any node will put the network out of action unless special precautions are taken, making a plain token ring unsuitable for industrial applications.

The ring can be modified in the following ways to improve reliability:

• Each node can have a set of bypass relay switches so that ring continuity is maintained even when a node fails or is switched off
• Ring architecture can be duplicated to provide two communication paths

Data frame

A token ring data frame has the structure shown in Figure 13.15.

Hardware requirements

The hardware layout for a token ring system is shown in Figure 13.16.

Although they use a logical ring structure, token rings are actually arranged in a hub (star) topology, with each node connected to a central hub or MAU (multistation access unit). MAUs can be interconnected by using special RI (ring in) and RO (ring out) ports on the MAUs.

Topology

The physical topology is that of a bus, but overlaying that is a logical ring topology (based on addresses) as illustrated in Figure 13.17.

Collision avoidance

Possession of a token is required before a node can transmit a frame, in the same way as described for token ring.

Data frame

A token bus data frame has the structure shown in Figure 13.18

Hardware requirements

The hardware components for a token bus node are shown in Figure 13.19.

Token bus configurations use a co-axial broadband cable, which has 75 Ω impedance. The cable is quite often quad shielded with a foil/braid/foil/braid composition around the signal wire and the dielectric, as protection under even the worst operating conditions.

Cable segments are linked using ‘F’ connectors.

Some token bus configurations use optical fibre as the transmission medium. This arrangement uses a star configuration in which the center of the star may be a node (active star) or a coupler (passive star). In an active star, each node in the star sends its transmissions to the central node, which then broadcasts the transmission to all the other connected nodes.

In a passive star, the coupler, or signal re-director, is created by fusing the fibers coming from each of the nodes. This fusion creates paths between all nodes, so that any transmission from a node will automatically reach all the other nodes.

Repeaters

A repeater is used to connect two segments of the same LAN, and simply retransmits an incoming signal. The repeater also carries out collision checking. Remote segments may each have a repeater, joined by a link; also a repeater may operate between different types of segment such as coaxial cable and optical fiber cable. Figure 13.20 illustrates the use of repeaters in an Ethernet LAN.

Bridges

A bridge connects two networks, or two segments of one network. It acts as a node as far as each side is concerned. The data link layer protocol has to be the same on each side, but the physical media can be different.

Bridges are more intelligent than repeaters and have software that ensures that noise and truncated packets are not passed across. The most commonly used bridges maintain address lists so that only packets addressed to the other side are retransmitted. In addition, they usually have ‘learning’ algorithms that allow them to build up and maintain complete address lists that can respond to changes in the network. Splitting a network with such bridges can produce a dramatic reduction in traffic density.

Another use for bridges is to extend a network. Two fully stocked networks (that is, with the maximum number of nodes and segments) can be joined by a bridge and made into effectively one large network. In fact it will be more efficient than an equivalent single network would be because the bridge ensures that only the necessary messages are repeated to one side or the other.

Switches

On a normal hub, all ports are interconnected and hence all users connected to that hub share the same available bandwidth. Any traffic on a given port will be ‘seen’ by all users connected to the hub.

A switching hub (or ‘switch’), on the other hand, only forwards each packet to the relevant port, based on the hardware address information in the header. A switch therefore acts as a multi-port bridge.

The advantage of using a switch to interconnect several LANs is that each LAN retains its full bandwidth, whilst still allowing communication between all hosts.

Routers

A router transfers data between networks that have the same network layer protocols (such as TCP/IP) but not necessarily the same physical or data link protocols. Routers maintain tables of addresses in the networks to which they are attached, and route each packet to the appropriate network depending on its destination address.

When a packet arrives at one side, the router translates the data link protocol if necessary, and then sends the packet on to the next node on the route. Figure 13.22 illustrates the use of routers.

Gateways

A gateway can connect networks that are completely dissimilar, and may need to translate all seven OSI protocol layers. Gateways therefore have the highest overhead and lowest performance of the internetworking devices.

Transmission control protocol/Internet protocol (TCP/IP)

This is the result of a US Department of Defense initiated project to implement a global network, interconnecting various local area networks or individual computers. In this sense this is a demonstrated open system model. The architecture is based on a four-layer model. The layers are:

• Application layer
  This comprises the session, presentation and application layers of the OSI model.
• Services layer (host-to-host)
  This represents the transport layer of the OSI model.
• Internetwork layer
  This represent the network layer of the OSI model.
• Network interface layer
  It represents the physical and datalink layers of the OSI model.

Figure 13.22 shows how TCP/IP relates to the OSI/RM. It can be seen that although the requirement for communication is the same or similar in reality, they are classified differently in different architectures. Hence it is not possible to define the exact equivalence of layers in different architectures.

Due to the support of the US Department of Defense and later by the National Science Foundation, TCP/IP has become the de facto open systems standard. It is widely used on the Internet and is supported by many PLC manufacturers for industrial interfacing.

Systems network architecture (SNA)

SNA is a layered architecture similar to OSI/RM. SNA is part of an IBM corporate design philosophy, which laid the framework for the data communication development of its products. The scope of SNA is so broad that none of its products implements the entire architecture. Each product implements only those elements of SNA that pertain to the function of that particular product. As SNA is a layered architecture, integration of all these products to form a network is only a matter of the proper configuration. The layers are described below.

• End user layer
  This describes the end user requirement of communication, much like the application layer of OSI.
• Function management layer
  All the requirements of translation in terms of coding or file formats, together with their management, are described in this layer and are functionally equivalent to the presentation layer of OSI.
• Data flow control layer
  This takes care of the control aspects of creating an end-to-end connection/ session.
• Transmission control
  This describes the end-to-end data transmission details such as reliability and integrity.
• Path control layer
  The actual sequencing of information packets and its routing on the network are described in this layer and is functionally equivalent to the transport and network layers put together of OSI.
• Data link layer
  This is essentially similar to that of OSI.
• Physical layer
  The description of the physical media details as in OSI.

SNA, when originally marketed, had only five layers and did not have end-user and physical layers. This was because all traditional mainframe operating systems provided the end user interface as part of the environment and the communication requirements were described without them. Also, since IBM had proprietary network standards, where the physical media details were described, there were no additional layers described. However, a seven layer integrated SNA model was proposed later and became an accepted standard. Other vendors had to reconcile with this and had to give SNA connectivity due to market pressures.

While OSI/RM is only a functional model, SNA is a functional as well as an implemented proprietary standard. It is dynamic and is growing in order to accommodate developments in the networking field. Advanced peer to peer communication (APPC) is the result of such a development and is primarily designed to provide enhanced capabilities for a distributed environment.

NETBIOS and NETBEUI

NETBIOS is an interface, an upper-level protocol developed by IBM. It provides a standard interface to the lower networking layers and functionally covers the top three layers (session, presentation and application) in the OSI reference model.

NETBIOS can also serve as an API (Application Program Interface) for data exchange. It provides programmers with access to resources for establishing a connection between two machines or between two applications on the same machine.

NETBIOS provides four types of service

• Naming, for creating and checking group and individual names (hardware or symbolic names)
• Datagram support, for connectionless communication
• Session support, for connection-orientated communication
• General services (e.g. resetting adapter states, canceling application commands, etc.)

NETBEUI (network basic extended user interface) is a protocol developed originally for use on IBM token ring networks. NETBEUI can communicate with standard (IEEE 802.2 LLC) protocols at the lower layers.

NETBEUI can be used instead of TCP/IP or SPX/IPX in a protocol stack. It is faster than TCP/IP and SPX/IPX and is ideally suited for small LANs but it is unfortunately not routable.

SPX/IPX

Novell has a major share of the commercial LAN market today with its NetWare product ranges. Comparison of the NetWare components SPX and IPX to OSI/RM is given in Figure 13.23.

A generalized number system

A number system is formed by allocating symbols to specific numerical values. Any group of symbols can be used with the total number of symbols for a number system called the base of the system.

The three most common bases are:

• Binary with two symbols (0 and 1) and hence a base of 2.
• Hexadecimal with sixteen symbols (0,1,2…9,A, B….F) and hence a base of 16.
• Decimal with ten symbols (0,1,2…9) and hence a base of 10.

When numbers with different bases are being used in the same descriptive text they sometimes have the subscript referring to the base being used, as in 3421.19₁₀ for a decimal or base 10 number.

Numerical symbols have to be combined in a certain way to represent other combinations of numbers. The decimal numbering system has the structure laid out in Table A.1 for weighting each digit in the number 3421.19₁₀ in a combination of numbers written together.

Exponential notation is used here, for example: 10² means 100 and 10^–3 means 0.001.