PLC (Programmable Logic Controller)
An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.
What it is
A PLC is a rugged industrial computer built to control machines and processes. It reads signals from the field, runs a stored program, and switches outputs to make things happen — reliably, in real time.
How it works
It runs a continuous scan: read all inputs, execute the program logic, then update all outputs — over and over, many times a second. Inputs come from sensors, switches, and signals; outputs drive contactors, valves, lamps, and drives.
An output's indicator LED follows the program's intention, not the physical output — which is why an output can show 'on' while the field device stays dead (a failed output, lost field supply, or broken wire).
Where it's used
Machine control, process plants, building services — anywhere sequenced or interlocked control is needed. I/O can be local or remote, often over a network.
Common faults: inputs not reading despite a signal present, outputs on but device dead, module faults, comms loss, and the controller dropping into a fault/stop state.
Safety first
Forcing or returning a PLC to run can move machinery. Confirm the area is safe and back up the program before changes.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
PLC input not reading despite a signal present
A field device is clearly providing a signal, but the PLC input bit/LED doesn't come on — the program never sees the input, so logic that depends on it won't run.
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
PLC in fault / stop mode (not running the program)
The PLC has stopped running its program — a fault LED is on or it's in STOP/PROG mode — so no I/O is being controlled and the machine is dead in a defined way.
PLC communication fault (network / remote I/O)
The PLC has lost communication with a device, remote I/O, HMI, or network — comms-fault indication, missing data, or remote I/O dropping out.
Related definitions
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Proximity sensor
Detects the presence of a target without contact — inductive, capacitive, or photoelectric.
HMI (operator panel)
A touchscreen/operator interface that lets people monitor and control a machine or process.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.