$3\mathrm{.}2$ Before the PLC was introduced around $1970,$ hardwired controllers composed of relays, coils, counters, timers, and similar components were used to implement this type of industrial control. Today, many older pieces of equipment have been retrofitted with PLCs to replace the original hardwired controllers, often making the equipment more productive and reliable than when it was new. Evaluate the four basic elements of the PLC$.$