PLC Overview

A Programmable Logic Controller (PLC) is an industrial computer specialized for real time applications. PLCs are integrated systems containing a processor, power supply, input modules, output modules and special purpose modules. Input modules interface with plant equipment and convert the field signals to logic levels for the processor to read. The processor uses these inputs to perform control functions based on application software. Output modules transmit the signals via an interface with the plant equipment. In addition there are special modules for communication with other computers, specialized dedicated functions, and conventional high-level language co- processors. Ladder logic will be used in the examples for the purpose of exposition.