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 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. 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,being able to be switched on or off and to switch other devices on or off. Hence the term internal relay. Such internal relays do not exist as real-world switching devices but are merely bits in the storage memory that behave in the same way as relays. For programming, they can be treated in the same way as an external relay output and input. Thus inputs to external switches can be used to give an output from an internal relay. This then results in the internal relay contacts being used, in conjunction with other external input switches, to give an output, such as activating a motor. Thus we might have (Figure 7.1) On one rung of the program: Inputs to external inputs activate the internal relay output. On a later rung of the program: As a consequence of the internal relay output, internal relay contacts are activated and so control some output. In using an internal relay, it has to be activated on one rung of a program and then its output used t...
