Programmable Logic Controller (PLC) — definition
Programmable Logic Controller (PLC) is an industrial digital computer designed to execute control logic reliably in real time for machinery and process automation in harsh factory environments.
PLCs read inputs from sensors and switches, execute a control programme, and drive outputs such as motors, valves and actuators. They are typically programmed in standardised languages such as ladder logic or structured text under IEC 61131-3 and are valued for ruggedness and deterministic timing.
Why it matters to industrial buyers
The PLC is the core decision-making device on most automated machines, so its programming, documentation and spare-parts availability affect long-term maintainability.
Key reference points
Standard languages
IEC 61131-3 defines standard PLC programming languages including ladder diagram and structured text.
Longevity
PLC hardware in industrial service commonly remains in operation for a decade or more before replacement.
Commonly confused with
industrial PC
A PLC is a ruggedised, deterministic controller for real-time machine logic; an industrial PC typically runs general-purpose operating systems for higher-level tasks such as visualisation or data processing.
How it is used in practice
The bottling machine's PLC coordinated conveyor speed, filling valves and reject mechanisms based on sensor feedback along the line.
Frequently asked questions
Is a PLC the same as a safety controller?
Not necessarily; standard PLCs handle general logic, while safety-rated PLCs or dedicated safety controllers are used for safety functions.
Can PLC programs be transferred between brands?
Generally not directly; program logic often needs re-engineering when changing PLC platforms, though IEC 61131-3 languages improve portability of concepts.
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Related terms
Supervisory Control and Data Acquisition (SCADA)
Supervisory Control and Data Acquisition (SCADA) is a software system that monitors and controls distributed industrial processes by collecting data from field controllers and presenting it for centralised operator oversight.
Human-Machine Interface (HMI)
Human-Machine Interface (HMI) is the operator-facing display and control panel that allows personnel to monitor status, adjust parameters and respond to alarms on a machine or process.
Industrial Automation
Industrial automation is the use of control systems, robotics and information technology to operate production equipment and processes with reduced manual intervention.
Functional Safety
Functional safety is the part of overall machine safety that depends on control systems, sensors and logic operating correctly to detect hazards and bring equipment to a safe state.
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Reference content only. Global B2B Group is independent of equipment manufacturers and financing institutions; definitions are provided for education and do not constitute engineering, financial or legal advice.