PLC

Programmable Logic Controller (PLC) is defined as digital electronic device, used to programmable memory to store instructions and implement functions such as logic, sequencing, timing, counting and arithmetic to control machines and processes.

A PLC has many "input" terminals, through which it interprets "high" and "low" logical states from sensors and switches. It also has many output terminals, through which it outputs "high" and "low" signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control. In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions.

PLCs are industrial computers, and as such their input and output signals are typically 120 volts AC, just like the electromechanical control relays they were designed to replace. Although some PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in logic gate circuits, this is the exception and not the rule.

PLC is a user friendly microprocessor based controller that carries out control functions of many types and levels of complexity. It is used extensively because it easy to setup and program, behaves predictably and rugged.