Instrumentation/computer networks
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This chapter discusses some aspects of the general concepts and design of networks, and this will prove useful in helping the reader to understand the mode of operation of existing networks. 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. 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. Three alternative forms of serial communication exist—simplex, half-duplex, and full-duplex. Parallel data buses allow much higher data rates than serial communication lines, because data is transmitted in parallel, that is, several bits are transmitted simultaneously. Local area networks transmit data in digital format along serial transmission lines. Highway Addressable Remote Transducer (HART) is a well-known, bus-based networking protocol that satisfies this need. “Fieldbus” is a genetic word that describes a range of high-speed, bus-based network protocols that support two-way communication in digital format between a number of intelligent devices in a local area network. Once a system gets too large to be covered by a local area network, it is generally necessary to use telephone lines. Network design and protocol are changing at a similar rapid rate to that of computer systems as a whole.