10 Instrumentation/computer networks

 10.1 Introduction

The inclusion of computer processing power in intelligent instruments and intelligent actuators creates the possibility of building an instrumentation system where several intelligent devices collaborate together, transmit information to one another and execute process control functions. Such an arrangement is often known as a distributed control system. Additional computer processors can also be added to the system as necessary to provide the necessary computational power when the computation of complex control algorithms is required. Such an instrumentation system is far more fault tolerant and reliable than older control schemes where data from several discrete instruments is carried to a centralized computer controller via long instrumentation cables. This improved reliability arises from the fact that the presence of computer processors in every unit injects a degree of redundancy into the system. Therefore, measurement and control action can still continue, albeit in a degraded form, if one unit fails.

In order to effect the necessary communication when two or more intelligent devices are to be connected together as nodes in a distributed system, some form of electronic highway must be provided between them that permits the exchange of information. Apart from data transfer, a certain amount of control information also has to be transferred. The main purpose of this control information is to make sure that the target device is ready to receive information before data transmission starts. This control information also prevents more than one device trying to send information at the same time.

In modern installations, all communication and data transmission between processing nodes in a distributed instrumentation and control system is carried out digitally along some form of electronic highway, although analogue data transmission (mainly current loop) is still widely used to transmit data from field devices into the processing nodes. If analogue transmission is used for measurement data, an analogue-to-digital converter must be provided at the interfaces between the measurement signal transmission cables and the processing nodes.

The electronic highway can either be a serial communication line, a parallel data bus, or a local area network. Serial data lines are very slow and are only used where a low data transmission speed is acceptable. Parallel data buses are limited to connecting a modest number of devices spread over a small geographical area, typically a single room, but provide reasonably fast data transmission. Local area networks are used to connect larger numbers of devices spread over larger geographical distances, typically a single building or site. They transmit data in digital format at high speed. Instrumentation networks that are geographically larger than a single building or site can also be built, but these generally require transmission systems that include telephone lines as well as local networks at particular sites within the large system.

The input/output interface of an intelligent device provides the necessary connection between the device and the electronic highway. The interface can be either serial or parallel.

A serial interface is used to connect a device onto a serial communication line. The connection is effected physically by a multi-pin plug that fits into a multi-pin socket on the casing of the device. The pins in this plug/socket match the signal lines used in the serial communication line exactly in number and function. Effectively, there is only one standard format for serial data transmission that enjoys international recognition. Whilst this is advantageous in avoiding compatibility problems when connecting together devices coming from different manufacturers, serial transmission is relatively slow.

A parallel interface is used to connect devices onto parallel instrument buses and also into all other types of network systems. Like the serial interface, the parallel interface exists physically as a multi-pin plug that fits into a multi-pin socket on the casing of the device. The pins in the plug/socket are matched exactly in number and function with the data and control lines used by a particular parallel instrument bus. Unfortunately, there are a number of different parallel instrument buses in use and thus a corresponding number of different parallel interface protocols, with little compatibility between them. Hence, whilst parallel data transmission is much faster than serial transmission, there are serious compatibility problems to be overcome when connecting together devices coming from different manufacturers because of the different parallel interface protocols used.

 

10.2 Serial communication lines

Serial communication only allows relatively slow data transfer rates, but it can operate over much larger distances than parallel communication. Transmission distances up to 3 km are possible with standard copper-wire links, and much greater distances can be achieved using either telephone lines or radio telemetry. Data are transferred down a single line on the electronic highway one bit at a time, and the start and finish of each item of data are denoted by special sequences of control characters that precede and follow the data bits.

Three alternative forms of serial communication exist, known as simplex, half-duplex and full-duplex. Simplex mode only allows transmission of data in one direction. For this reason, it is not widely used, since, although it permits a remote sensor to transmit information, the receiving station cannot send a message back to acknowledge receipt or request retransmission if the received data has been corrupted. In half-duplex mode, the same data wire is used by a device to both send and receive data, and thus the receiving station is able to acknowledge receipt of data from a sensor. However, sending and receiving of data simultaneously is not possible. In full-duplex operation, two separate data lines are used, one for send and one for receive, and so simultaneous sending and receiving of data is therefore possible. In addition to these three forms (simplex, half-duplex and full-duplex), two different transmission modes exist, known as asynchronous transmission and synchronous transmission.

10.2.1 Asynchronous transmission

The structure of the data and control characters used in asynchronous transmission are shown in Figure 10.1. The binary digits of ‘1’ and ‘0’ are represented by voltage logic levels of +V and zero. The start of transmission of each character is denoted by a binary ‘0’ digit. The following seven digits represent a coded character. The next digit is known as a parity bit. Finally, the end of transmission of the character is denoted by either one or two stop bits, which are binary ‘1’ digits.

The parity bit is provided as an error checking mechanism. It is set to make the total number of binary ‘1’ digits in the character representation either odd or even, according to whether the odd or even parity system is being used (some manufacturers use odd parity and some use even parity). The seven character digits are usually coded using the ASCII system (American Standard Code for Information Interchange). This provides for the transmission of the full set of alphabetic, numerical, punctuation and control characters.

Asynchronous transmission allows the transmitter and receiver to use their own clock signals to put data on to the transmission line at one end and take it off at the other end. Receipt of the start bit causes the receiver to synchronize its clock with the incoming data, and this synchronization is maintained whilst the stream of character bits, parity bit and stop bit(s) are received. One particular disadvantage of asynchronous transmission

is that transfer only occurs one bit at a time, and so the data transfer efficiency is low. Also, only seven out of each ten or 11 bits transmitted represent a character, with the other three or four bits being for synchronization. Maximum transmission speed is 19 200 bit/s, which is frequently referred to as a speed of 19.2 kbaud (where 1 baud D 1 bit/s).

However, although the data transfer rate is slow, asynchronous serial transmission does have the advantage that only two different standard formats exist, and these are very similar. These two formats, which have achieved international recognition, are the RS232 standard (USA) and the CCITT V24 standard (European). The only significant difference between these is the logic voltage level used, 3 V for RS232 and 6 V for V24, and this incompatibility can be handled very easily. Within either of these standards, there are options about the type of parity (odd or even), the number of stop bits and whether data transmission is in full- or half-duplex mode. The various equipment manufacturers use the options differently but it is a relatively simple matter to accommodate these differences when connecting together devices coming from different manufacturers.

When asynchronous transmission is not fast enough, local area networks are used that transmit data synchronously. This is covered in detail in section 10.4.