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What is a PLC?
PLC is short for programmable logic controller. A PLC is made up of two basic components. The first is the central processing unit or CPU, which controls the activity of the PLC. The CPU, itself, is composed of two parts, the processor and the memory. The second component of the PLC is the input/output interface or commonly know as I/O. Field devices such as push buttons, micro-switches, proximity sensors, and numerous other devices are wired to the input terminals of the PLC. Relays, contactors, solenoids, and indicator lights are just some of the devices that can be wired to the output terminals of the PLC. These terminals interface the various devices to the CPU for control by the program written in the processor.
The CPU receives the information from the inputs, performs the functions directed in the program, and changes the outputs according to the inputs and program. The processor continuously scans the program, adjusting the outputs almost instantaneously with changes in the inputs.
The programs are created, usually on a computer using a programming language that is specific to that family of PLCs. This program is next downloaded to the PLC, providing the CPU with its directions on how to deal with the inputs and outputs that have been connected to the PLC.
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