7 Internal relays

This chapter continues on from the previous chapters on programming and introduces internal relays. A variety of other terms are often used to describe these elements, e.g. auxiliary relays, markers, flags, coils, bit storage. These are one of the elements giving special built-in functions with PLCs and are very widely used in programming. A small PLC might have a hundred or more internal relays, some of them being battery backed so that they can be used in situations where it is necessary to ensure safe shutdown of plant in the event of power failure. Later chapters consider other common built-in elements.

7.1 Internal relays

In PLCs there are elements that are used to hold data, i.e. bits, and behave like relays, being able ...

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