A programmable logic controller (PLC) is an industrial controller designed to execute deterministic discrete-control logic using field inputs and outputs.

A basic PLC repeatedly performs a scan cycle:

read inputs
    ↓
execute control program
    ↓
update outputs
    ↓
repeat

This means logic usually operates on an input image captured for the scan rather than on infinitely fast continuous changes.

Ladder logic represents Boolean control using horizontal rungs. Contacts represent conditions and coils represent results. A normally open contact is true when its associated Boolean variable is true; a normally closed contact represents the logical negation of that variable.

A simple motor command can be expressed conceptually as

Enable AND GuardClosed AND NOT Fault  -> MotorOn

If all three permissive conditions are satisfied, the output command becomes true. If Fault becomes true, the negated condition becomes false and the command drops on the next program evaluation.

PLC programs also use timers, counters, memory bits and structured sequence logic, but these build on the same cycle of observing inputs, updating internal state and commanding outputs.

The scan model matters when debugging. A variable changed by one rung can affect later logic during the same program execution, while a physical input generally changes the program's view only when the input image is refreshed according to controller behavior.

PLCs therefore turn Boolean and sequential logic into robust industrial control over sensors, valves, contactors and other machine interfaces.