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 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