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programmable logic controller (PLC)

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. The first PLC, for example, was invented by Dick Morley in 1969 for General Motors and performed uninterrupted for 20 years before being retired.

Five programming languages are used to code PLCs, as specified by International Electrotechnical Commission (IEC) 61131. They are Ladder Logic, Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL) and Sequential Function Chart (SFC). Should significant code changes be required and the PLC's memory is embedded, the controller can be recoded in place. When the PLC's memory is not embedded on the circuit board and significant code changes are required, the memory can be removed from an exterior slot on the PLC and replaced without requiring the assistance of a programmer on site. 

This was last updated in March 2016

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