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A programmable logic controller (PLC) or programmable controller is

[a] microprocessor based controller capable of accepting input signals, performing pre-programmed digital control logic or analog control action, and providing output signals. [1]
[a] solid-state control system that has a user-programmable memory for storing instructions for the purpose of implementing specific functions such as I/O control, logic, timing, counting, three mode (PID) control, communication, arithmetic, and data and file processing.[2]
[a] digital computer used for the automation of electromechanical processes.[3]


PLCs are used in many industries and machines, such as packaging and semiconductor machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real-time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.


  1. Department of the Army, Supervisory Control and Data Acquisition (SCADA) Systems for Command, Control, Communications, Computer, Intelligence, Surveillance, and Reconnaissance (C4ISR) Facilities, at B-4 (Jan. 21, 2006) (full-text).
  2. NIST Special Publication 800-82, at B-6.
  3. Cybersecurity A Primer for State Utility Regulators, App. B.

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