Way back, before Programmable Logic Controllers (PLCs) became the norm, plant floor automation was performed with racks and racks of industrial relays, pneumatic plunger timers, and electromagnetically counters to control the starting and stopping of motors, opening of valves, and other control-related process interactions. The program that ran the control for such a setup was not a program at all but a combination of interconnected circuits, timers, and relays. By forming the electrical paths, physical actions such as opening valves, running motors, and turning on lights were accomplished. The programmer of a relay system like this was the plant floor electrical engineer and program changes involved physically ...
Industrial control system history
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