10 Instrumentation/computer networks

 10.4 Local area networks (LANs)

Local area networks transmit data in digital format along serial transmission lines. Synchronous transmission is normally used because this allows relatively high transmission speeds by transmitting blocks of characters at a time. A typical data block consists of 80 characters: this is preceded by a synchronization sequence and followed by a stop sequence. The synchronization sequence causes the receiver to synchronize its clock with that of the transmitter. The two main standards for synchronous, serial transmission are RS422 and RS485. A useful comparison between the performance and characteristics of each of these and RS232 (asynchronous serial transmission) can be found in Brook, (1996).

LANs have particular value in the monitoring and control of plants that are large and/or widely dispersed over a large area. Indeed, for such large instrumentation systems, a local area network is the only viable transmission medium in terms of performance and cost. Parallel data buses, which transmit data in analogue form, suffer from signal attenuation and noise pickup over large distances, and the high cost of the long, multi-core cables that they need is prohibitive.

The development of instrumentation networks is not without problems, however. Careful design of the network is required to prevent corruption of data when two or more devices on the network try to access it simultaneously and perhaps put information onto the data bus at the same time. This problem is solved by designing a suitable network protocol that ensures that network devices do not access the network simultaneously, thus preventing data corruption.

In a local area network, the electronic highway can take the form of either copper conductors or fibre-optic cable. Copper conductors are the cheapest option and allow transmission speeds up to 10 Mbit/s, using either a simple pair of twisted wires or a coaxial cable. However, fibre-optic cables are preferred in many networks for a number of reasons. The virtues of fibre-optic cables as a data transmission medium have been expounded in Chapter 8. Apart from the high immunity of the signals to noise, a fibreoptic transmission system can transfer data at speeds up to 240 Mbit/s. The reduction in signal attenuation during transmission also means that much longer transmission distances are possible without repeaters being necessary. For instance, the allowable distances between repeaters for a fibre-optic network are quoted as 1 km for half-duplex operation and up to 3.5 km for full-duplex operation. In addition, the bandwidth of fibre-optic transmission is higher than for electrical transmission. Some cost saving can be achieved by using plastic fibre-optic cables, but these cannot generally be used over distances greater than about 30 m because signal attenuation is too high.

There are many different protocols for local area networks but these are all based on one of three network structures known as star networks, bus networks and ring networks, as shown in Figure 10.3. A local area network operates within a single building or site and can transmit data over distances up to about 500 m without signal attenuation being a problem. For transmission over greater distances, telephone lines are used in the network. Intelligent devices are interfaced to the telephone line used for data transmission via a modem. The modem converts the signal into a frequency-modulated analogue form. In this form, it can be transmitted over either the public switched telephone network or over private lines rented from telephone companies. The latter, being dedicated lines, allow higher data transmission rates.

 

10.4.1 Star networks

In a star network, each instrument and actuator is connected directly to the supervisory computer by its own signal cable. One apparent advantage of a star network is that data can be transferred if necessary using a serial communication protocol such as

RS232. This is an industry standard protocol and so compatibility problems do not arise, but of course data transfer is very slow. Because of this speed problem, parallel communication is usually preferred even for star networks.

Whilst star networks are simple in structure, the central supervisory computer node is a critical point in the system and failure of this means total failure of the whole system. When any device in the network needs to communicate with another device, a request has to be made to the central supervisory computer and all data transferred is routed through this central node. If the central node is inoperational for any reason then data communication in the network is stopped.

 

10.4.2 Ring and bus networks

In contrast, both ring and bus networks have a high degree of resilience in the face of one node breaking down. Hence, they are generally preferred to star networks. If the processor in any node breaks down, the data transmission paths in the network are still maintained. Thus, the network can continue to operate, albeit at a degraded performance level, using the remaining computational power in the other processors. Most computer and intelligent instrument/actuator manufacturers provide standard conversion modules that allow their equipment to interface to one of these standard networks.

In a ring network, all the intelligent devices are connected to a bus that is formed into a continuous ring. Ring protocol sends a special packet (or token) continuously round the ring to control access to the network. A station can only send data when it receives the token. During data transmission, the token is attached to the back of the message sent so that, once the information has been safely received, the token can continue on its journey round the network. A typical data transmission speed is 10 Mbit/s. Cambridge Ring, Arcnet and the IEEE 802.5 bus are examples of token ring protocols.

A bus network is similar to a ring network but the bus that the devices are connected onto is not continuous. Bus networks are also resilient towards the breakdown of one node in the network. A contention protocol is normally used. This allows any station to have immediate access to the network unless another station is using it simultaneously, in which case the protocol manages the situation and prevents data loss/corruption. They have a similar data transmission speed to ring networks of 10 Mbit/s. Ethernet and the IEEE 802.3 standard bus are examples of bus networks.

 

10.5 Gateways

Gateways, such as P1451 produced by the IEEE, are interfaces between intelligent devices and local area networks that overcome the non-compatibility problem between buses using different protocols. As a different gateway is required for each different LAN that a device may be connected to, this theoretically adds cost to the system and imposes a time delay that reduces performance. However, the availability of fast processing power at low cost means that the use of a gateway is a feasible solution to the problem of using devices from different suppliers that are designed for different buses. Alternative forms of gateway also provide a means of connecting analogue devices into a digital network, particularly those using 4–20 mA current loop transmission standards. In many cases, gateways provide a means of retaining existing equipment in a new digital network and thus avoid the expense of buying new devices throughout a plant.

 

10.6 HART

As intelligent devices developed over the years, the need arose for network protocols that could provide for the necessary digital communications to and from such devices. HART (Highway Addressable Remote Transducer) is a well-known bus-based networking protocol that satisfies this need. Over the years, this has gained widespread international use, and has now become a de facto standard, with HART-compatible devices being available from all major instrument manufacturers. Recent surveys have predicted that HART will continue in widespread use for the next 15 to 20 years, irrespective of the timing of the long-promised, internationally accepted, all-digital fieldbus standard.

HART was always intended to be an interim network protocol to satisfy communication needs in the transitional period between the use of analogue communication with non-intelligent devices and fully digital communication with intelligent devices according to an international standard digital fieldbus protocol. Because of this need to support both old and new systems, HART supports two modes of use, a hybrid mode and a fully digital mode.

In hybrid mode, status/command signals are digital but data transmission takes place in analogue form (usually in 4–20 mA format). One serious limitation of this mode is that it is not possible to transmit multiple measurement signals on a single bus, since the analogue signals would corrupt each other. Hence, when HART is used in hybrid mode, the network must be arranged in a star configuration, using a separate line for each field device rather than a common bus.

In fully digital mode, data transmission is digital as well as status/command signals. This enables one cable to carry signals for up to 15 intelligent devices. In practice, the fully digital mode of HART is rarely used, since the data transmission speed is very limited compared with alternative fieldbus protocols such as Profibus. Therefore, the main application of the HART protocol has been to provide a communication capability with intelligent devices when existing analogue measurement signal transmission has to be retained because conversion to fully digital operation would be too expensive.