A PLC (programmable logic controller) is a rugged industrial computer whose main job is to make the same control decision, predictably, thousands of times a minute. A useful mental model is: sense -> decide -> act -> repeat.
During each scan the PLC normally:
Reads its physical inputs into an input image.
Executes the control program (ladder logic, structured text, function blocks, etc.).
Copies the calculated output image to the physical outputs.
Runs diagnostics and communications, then starts the next scan.
A scan often takes only a few milliseconds. Reading all inputs as a snapshot before solving the logic also makes the behavior easier to reason about than a general-purpose program reacting at arbitrary times.
Typical inputs include:
Digital: a Start button, limit switch, photoelectric sensor, or overload contact (ON/OFF).
Analog: temperature, pressure, flow, or level, commonly represented as 4-20 mA or 0-10 V.
Typical outputs include:
Digital: a warning lamp, solenoid valve, contactor, or relay coil.
Analog: a speed reference for a variable-frequency drive or a valve-position command.
Simple conveyor example: pressing Start does not directly power the motor. The PLC sees the Start input, checks interlocks such as "guard closed," "emergency stop healthy," and "motor not overloaded," then energizes the motor output. A photoeye at the end detects a box; the program can stop the belt, actuate a pneumatic pusher for one second, retract it, and restart the belt. Timers and sequence state make every cycle repeat consistently. If an interlock becomes false, the normal outputs drop to their defined safe state.
PLCs are used because their I/O is electrically isolated, their scan timing is deterministic, they tolerate heat/noise/vibration better than office computers, and technicians can troubleshoot live logic and I/O without rewriting an entire application. One important boundary: personnel-safety functions should use safety-rated relays or a safety PLC and an engineered safety circuit, not ordinary application logic alone.
A PLC (programmable logic controller) is a rugged industrial computer whose main job is to make the same control decision, predictably, thousands of times a minute. A useful mental model is: sense -> decide -> act -> repeat.
During each scan the PLC normally:
A scan often takes only a few milliseconds. Reading all inputs as a snapshot before solving the logic also makes the behavior easier to reason about than a general-purpose program reacting at arbitrary times.
Typical inputs include:
Typical outputs include:
Simple conveyor example: pressing Start does not directly power the motor. The PLC sees the Start input, checks interlocks such as "guard closed," "emergency stop healthy," and "motor not overloaded," then energizes the motor output. A photoeye at the end detects a box; the program can stop the belt, actuate a pneumatic pusher for one second, retract it, and restart the belt. Timers and sequence state make every cycle repeat consistently. If an interlock becomes false, the normal outputs drop to their defined safe state.
PLCs are used because their I/O is electrically isolated, their scan timing is deterministic, they tolerate heat/noise/vibration better than office computers, and technicians can troubleshoot live logic and I/O without rewriting an entire application. One important boundary: personnel-safety functions should use safety-rated relays or a safety PLC and an engineered safety circuit, not ordinary application logic alone.