A PLC, programmable logic controller, is a rugged industrial computer that runs machinery. It reads sensors, executes control logic, and switches outputs (motors, valves, lights) in a repeating cycle measured in milliseconds. PLCs replaced hardwired relay panels and are the standard control brain in nearly every factory built or upgraded since the 1970s.

If SCADA is the control room and MES is the production office, the PLC is the reflex at the machine: fast, deterministic, unglamorous, and utterly load-bearing. Everything the modern plant wants to do with data, machine monitoring OEE, analytics, ultimately traces down to signals a PLC is already reading.

Where did the PLC come from?

From a car company's frustration with relays. Through the 1960s, machine control meant cabinets full of electromechanical relays, hardwired into logic. Every model-year change meant rewiring cabinets by hand, weeks of downtime, error-prone, and documented mostly in electricians' heads. In 1968, General Motors' Hydra-Matic transmission division issued a request for proposals for a programmable, computer-like replacement for hardwired relay systems, based on a white paper by engineer Edward R. Clark.

The winning proposal came from Bedford Associates of Bedford, Massachusetts, where engineer Dick Morley had written a concept memo dated January 1, 1968, often cited as the PLC's birth certificate. The prototype was the firm's 84th project, so the machine became the 084; the venture spun into a company named Modicon (for MOdular DIgital CONtroller), and GM took delivery of its first units in November 1969 (Control Engineering; Dick Morley). Morley, who died in 2017, is remembered as the father of the PLC. The design brief from 1968 still describes the product: survive the factory floor, be programmable by plant electricians rather than computer scientists, and never, ever stop.

How does a PLC work? The scan cycle

A PLC does one thing in an endless loop, and the loop is the key to understanding it.

The PLC scan cycle1 · READ INPUTSsnapshot every sensor2 · EXECUTE LOGICrun the program top to bottom3 · UPDATE OUTPUTSwrite coils, motors, valves4 · HOUSEKEEPINGdiagnostics + commsone full scan:~1-50 msthen repeat, forever
The scan cycle. The PLC snapshots all inputs, runs the whole program against that snapshot, writes all outputs at once, then repeats, typically every 1–50 milliseconds.

Three properties fall out of this loop. Determinism: the program runs top to bottom against a frozen snapshot of inputs, so behavior is repeatable, the same conditions always produce the same outputs, on a known schedule. Predictable timing: scan time is measured and bounded; if the emergency condition appears, the response comes within a scan, not "whenever the operating system gets around to it." Robustness: there is no desktop OS underneath to update, fragment, or crash. This is why a thirty-year-old controller in your compressor room has probably rebooted fewer times than your laptop has this month.

What is ladder logic?

The dominant PLC programming language, and a deliberate act of user-interface genius. Because PLCs replaced relay panels, their logic was drawn to look like relay wiring diagrams, two vertical power rails with horizontal "rungs" of contacts and coils, so the electricians who maintained relay panels could read and troubleshoot the software the same way they read the old drawings.

A classic ladder rung: start/stop with seal-inSTART (NO)STOP (NC)MMOTOR (coil)MOTOR aux (NO)"seal-in": keeps rung truePress START → motor runs and holds itself in. Press STOP → rung breaks, motor drops out.
The classic first rung every PLC student learns: start/stop with seal-in. The motor's own auxiliary contact holds the circuit closed after the start button is released.

Reading the rung: current flows (logically) from the left rail through contacts to the coil on the right. The START contact is normally open, pressing the button closes it. The STOP contact is normally closed, it passes current until someone presses stop. When the motor coil energizes, its auxiliary contact (the parallel branch) closes and "seals in" the circuit, so the motor keeps running after START is released. Press STOP and the rung breaks, the coil drops, the seal-in opens. Safety lives in the details: STOP is wired normally closed so a broken wire stops the machine rather than making it unstoppable, fail-safe by construction. Ladder is one of five languages standardized in IEC 61131-3; the others (function block, structured text, and the rest) matter, but ladder still dominates U.S. floors because troubleshooting speed at 3 a.m. is the metric that counts.

What does the I/O side look like?

I/O wiring concept: sensors in, logic in the middle, actuators outPHOTO EYEPUSH BUTTONLIMIT SWITCHTEMP PROBEINPUTMODULESCPUlogicOUTPUTMODULESMOTOR STARTERSOLENOID VALVESTACK LIGHTSensors in the left, decisions in the middle, actuators out the right, thousands of times a minute.
The I/O concept: field devices wire into input modules, the CPU decides, output modules drive the actuators. Modern PLCs add communication ports, which is where monitoring taps in.

Physically, a PLC is a CPU plus racks of input and output modules. Discrete inputs are on/off signals (buttons, limit switches, photo eyes); analog inputs carry continuous values (temperature, pressure, speed) typically as 4–20 mA or 0–10 V signals. Outputs mirror the split: discrete outputs fire motor starters, solenoids, and stack lights; analog outputs command drives and valves. Modern PLCs also expose communication ports, and those ports are how the rest of the plant gets its data. An edge gateway subscribing read-only to PLC tags is the cheapest, least invasive source of machine truth in existence, which is why it is the first move in most IIoT retrofits: the sensors are already wired, the signals already exist, and control logic never gets touched. That is also where a layer like Harmony starts when connecting machines, read from the PLCs and sensors the plant already owns, compute true OEE from source signals, and build visibility and automation above the control layer. No rip-and-replace (connected systems module).

PLC vs PC vs microcontroller: which runs what?

The honest summary: PLCs win on the floor not because they are powerful but because they are boring in exactly the right ways predictable, rugged, maintainable by the people already in the building.

Key dates and numbers

  1. January 1, 1968: Dick Morley's concept memo at Bedford Associates, the earliest documented PLC design record (Morley interview, Manufacturing Automation).
  2. 1968: GM Hydra-Matic issues its RFP for a programmable relay replacement, from Edward R. Clark's white paper (Control Engineering).
  3. November 1969: first Modicon 084 units delivered to GM; the name Modicon stands for MOdular DIgital CONtroller (PLC history).
  4. IEC 61131-3: the international standard defining PLC programming languages, ladder diagram among them (IEC 61131-3).
  5. Typical scan times: roughly 1–50 milliseconds for a full read-execute-write cycle on production controllers.

Fifty-plus years on, the PLC's job description hasn't changed, but its data has never been more valuable. The controllers on your floor are already measuring almost everything the plant argues about in meetings. The step most plants are actually missing is not new controls; it is reading what the existing ones already know. For where that data goes next, see SCADA and the full smart factory stack.