Programmable Logic Controller (PLC)

A programmable logic controller is a ruggedised industrial computer that reads inputs from sensors, executes a stored control program in a repeating cycle and drives outputs such as motors, valves and indicators to automate machines and processes.

A PLC combines a processor, memory, a power supply and input/output modules in a housing built to tolerate the electrical noise, vibration and temperatures found on factory floors. It runs its user program in a continuous loop known as the scan cycle: inputs are read, logic is executed and outputs are updated. Programs are written on an engineering workstation, usually in the languages defined by IEC 61131-3, and downloaded to the controller.

PLCs were introduced in the late 1960s to replace hard-wired relay panels in automotive plants, where every production change previously meant rewiring. Today they control conveyors, packaging machines, pumps, presses, robot cells and process skids across almost every manufacturing sector. They exchange data with HMIs, SCADA systems, drives and other controllers over fieldbuses and industrial Ethernet, and occupy the basic control level (Level 1) of the Purdue model.

Selection depends on I/O count, required scan time, communication protocols, memory and environmental ratings. IEC 61131-2 covers PLC equipment requirements and tests, while IEC 61131-3 defines the programming languages. Standard PLCs are not safety controllers; safety functions require certified safety PLCs or safety relays designed to standards such as IEC 61508, IEC 62061 or ISO 13849. Program changes on running equipment should follow site change control, with OEM documentation and qualified personnel governing the work.

Key points

Executes a stored program cyclically: read inputs, solve logic, write outputs
Introduced in the late 1960s to replace hard-wired relay control panels
Programmed in IEC 61131-3 languages such as ladder logic and structured text
Equipment requirements and tests are covered by IEC 61131-2
Safety functions need certified safety controllers, not standard PLCs

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Programmable Logic Controller (PLC)

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