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Programmable Logic Controllers, 5th Edition by William Bolton

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Chapter 7. Internal Relays
This chapter continues from the previous chapters on programming and introduces internal relays. A variety of other terms are often used to describe these elements, such as auxiliary relays, markers, flags, coils, and 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, that is, bits, and behave like relays, ...

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