A programmable logic controller (PLC) is a small, rugged computer that sits beside a machine, reads the machine's sensors and switches the machine many times a second. It is built to survive heat, dust and vibration on a factory floor. It suits factories that make separate items, such as cars, because each item moves from one machine to the next in steps.
A PLC repeats one cycle again and again. It reads all its inputs, such as sensors and switches. It runs the program stored in it. It then sets its outputs, such as motors and valves. One cycle usually takes a few milliseconds, so the machine reacts almost at once. The program is written by an automation engineer and stored in the PLC's memory. It runs on the PLC's firmware, the permanent software that the maker installs.
The most common language for programming a PLC is called ladder logic. A ladder program looks like an electrical circuit diagram, with switches and coils drawn between two rails. This is deliberate. Plant electricians can read the diagram and find a fault, even if they are not programmers. Most PLCs also accept structured text, a language written as lines of code, like ordinary programming.
Consider a conveyor carrying a car door to a welding station in a factory near Chennai. A sensor tells the PLC that the door has reached the station. The PLC stops the conveyor and tells the welding robot to start. When the robot reports that it has finished, the PLC starts the conveyor again. The PLC does this for every door, all day.
The buyers are mostly machine builders. A company that makes a packing machine fits a PLC into every machine it sells, and the plant that buys the machine inherits the maker's choice. The plant then tends to buy the same brand again, so that its engineers need to learn only one. PLCs are rarely replaced. A PLC bought today may run the same program for twenty years. So engineers keep learning old models, and security teams have to protect them with OT security instead of replacing them.
The PLC takes its readings from industrial sensors. It sets the speed of a motor through a variable frequency drive and holds a heater at its set temperature through temperature controllers. It reports its status upwards to SCADA. A distributed control system does a similar job, but for continuous processes such as a refinery, where the product keeps moving without a stop.