Industrial Communication Guide

3. FIELDBUS SYSTEMS 3.1. FUNCTIONING OF A FIELDBUS

The majority of fieldbuses are based on the so-called master-slave method. Here the master takes on a role similar to that of the chair- person in a discussion, determining who is able to communicate and when. The sensors and actuators are designated as slaves and have a unique address. The master itself is usually contained in the PLC. A temperature-controlled fan explains how it works: The master first interrogates the sensor by requesting the sensor via message (1). The sensor then sends the current temperature value as a message (2) to the master, which is then processed in the controller. The master then sends another message (3) to the actu- ator containing the fan speed value. Since a fieldbus is operated cy- clically like a PLC, the master then starts to query the sensor again.

3.2. ADVANTAGES AND DISADVANTAGES OF THE FIELDBUS AS COMPARED TO CONVENTIONAL WIRING Advantages of the fieldbus • Easy to install: less wiring and smaller, simpler switch cabinets • Reduced error search in the event of failure • One cable for digital/binary and analog signals • Protection from faults with analog values • Automated system is capable of self-diagnosis – e.g. when faults occur in the sensors/actuators • Simpler expansion or simple addition of sensors/actuators

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