Newnes Interfacing Companion

2.7

Programmable logic controllers

2.7.1 Programmable logic controllers

Industrial processes have been traditionally controlled by electromechanical systems (i.e. switches and relays). An electromechanical control system controls the output states according to the input states. The logic behind the switching and the resultant actions depend upon way the switches and relays are wired together. The overall function in these systems is not so easily changed and the systems are not easily maintained.

In the late 1960s, digital controllers were introduced to allow some degree of programming to control the sequence of operations required in an industrial process. A programmable logic controller (PLC) examines its input states and turns off or on ...