Welcome to PLCTutor.com

Welcome to PLCTutor. This site exists to provide basic information about Programmable Logic Controllers without a bias toward any particular manufacturer.

What is a PLC?

NEMA, the National Electrical Manufacturers Association, defines a programmable logic controller (PLC) as:

a programmable controller is a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process.


Programmable Logic Controllers, programmable controllers, or PLCs are specialized industrial computers. The PLC accepts inputs from switches and sensors (measures or senses the system), evaluates these based on a program (logic), and changes the state of outputs to control a machine or process.


PLC System

Initially, programmable logic controllers were used to replace traditional hard-wired relay logic; however, with its ever increasing functionality it is found in many more complex applications. PLCs are used in any industrial application where operating requirements are complex, are constantly changing, or where high reliability is necessary.

Glossary of PLC Terms