A programmable logic controller (PLC) is a small, modular solid state computer with customized instructions for performing a particular task. PLCs, which are used in industrial control systems (ICS) for a wide variety of industries, have largely replaced mechanical relays, drum sequencers and cam timers.

PLCs are useful tools for repeatable processes because they have no mechanical parts and they can gather information. Each central processor unit (CPU) continually loops through an input scan, program scan, output scan and housekeeping mode, repetitively performing a single task while monitoring conditions. The information the controller gathers can be used as feedback to guide needed changes and improvements to processes, some of which can be performed automatically according to the device’s coding.

PLCs take up less space, perform more complex tasks and are more customizable than the mechanical technologies they have replaced. They are known for their ability to operate continuously without maintenance and have had a great impact on digitizing a great many industries, particularly manufacturing.