Industrial Engineer · phase 1 of 16

PLC

Programmable Logic Controller, an industrial computer that executes deterministic control logic for machines and processes.

The Explain card

Plain English
A PLC is a rugged computer built to run a machine. It reads its inputs, runs the logic you wrote, and writes its outputs, over and over, many times a second.
Example
A conveyor line has a start button, a jam sensor and a motor starter. The PLC watches the button and the sensor and decides, every few milliseconds, whether the starter coil stays on.
Why it matters
The same box replaces racks of relays and timers, and the logic can be changed from a laptop without rewiring a panel. Every deterministic scan is the promise a plant leans on.
Hook
A relay cabinet that learned to take instructions.

Word knowledge

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

The packaging PLC writes a 24 V DC output to the motor starter coil once the E-stop circuit and the infeed photo-eye are both made.

Letter by letter
  • P Programmable a stored program, not a panel you rewire
  • L Logic the same word as in ladder logic
  • C Controller the plant-floor title, kept after computer came to mean a desk machine
Where it came from
Origin
English industrial coinage from programmable controller. Bedford Associates built the Modicon 084 in 1968; Allen-Bradley inserted Logic so the title would not collide with the personal computer.
Entered the language
early 1970s
What changed
Early catalogs used PC. After the personal computer took those two letters, shops standardized on PLC, even though IEC 61131 still titles the class programmable controllers.
How it is spelled
Capitalisation
Written PLC in three capitals even mid-sentence; lowercase plc is the British company suffix, not this term.
Plural and possessive
The plural is PLCs; the possessive is PLC's. Never use PLC's for more than one.
Out loud
Spelled out letter by letter.

Same family

  • PAC
  • DCS
  • HMI
  • RTU
Broken into chunks
  • program
    • programmable
    • programmer
  • logic
    • logical
    • logician
  • control
    • controller
    • uncontrolled
What it sits beside

Same subject

Same shape

Plant controllers

Plant-floor initialisms

Where it sits in the deck

Phase 1: PLC Controller Hardware

The box itself, from processor and rack to the modules that plug into it, because nothing else in the deck lands until you know what a PLC physically is.

Neighbours in this phase