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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.
The program in a PLC responds to inputs with customized outputs. A simple program could maintain the level of water▼
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in a tank between two float switchs by opening and closing an electric valve. A slightly more complex arrangement could involve a scale under the tank (as an input) and a flow controller (as an output) allowing water to flow at a controlled rate. A typical industrial application might control several tanks in a process such as sewage treatment. Each tank might be watched for a variety of conditions such as being too full or too empty or having the wrong pH.
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