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PLCs often replace bulky cabinets full of electro-mechanical relay logic. Where older automated systems would use hundreds or thousands of relays, a single PLC can be programmed as a replacement.
The PLC is a [[microprocessor]] based device with either modular or integral [[input/output]] circuitry that monitors the status of the field connected "sensor" inputs (pushbuttons, limit-switches, photo-eyes, analog transmitters, etc.) and controls the attached output "actuators" (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created, logic program. The program is generally created using "relay ladder logic" development software in a PC and then downloaded and stored in the microprocessor's battery-backed [[RAM]] memory. The functionality of the PLC has evolved over the years to include capabilities beyond typical relay control; sophisticated motion control, process control, [[DCS]] and complex networking have now been added to the PLC's list of functions.
A simple program could maintain the level of water
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