Abstract
This chapter discusses 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 included among the 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 battery backed so that they can be used in situations where it is necessary to ensure safe shutdown of a plant in the event of power failure. With ladder programs, an internal relay output is represented using the symbol for an output device, namely ( ), with an address that indicates that it is an internal relay. The internal relay switching contacts are designated with the symbol for an input device and given the same address as the internal relay output. Internal relays that are battery-backed are able to retain their setting, even when the power is removed. The relay is said to be retentive.
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