A PLC (Programmable Logic Controller) is a rugged computer that repeatedly reads what is happening in a machine, makes decisions using a stored control program, and commands the machine's devices. Unlike an office PC, it is built for electrical noise, vibration, heat and continuous operation.
Its normal scan cycle is:
Read inputs. The PLC copies the current state of every connected sensor into an internal input image.
Execute logic. It evaluates the program from top to bottom using that snapshot. The program may be ladder logic, structured text, a function-block diagram or a sequential-function chart.
Update outputs. It writes the calculated results to an output image and then energizes or changes the real outputs.
Housekeeping. It performs communications and diagnostics, then repeats the cycle—usually every few milliseconds.
Typical inputs include a pushbutton, limit switch, photoelectric sensor, pressure switch and emergency-stop status. Analog inputs represent a range rather than only on/off: for example, a 4–20 mA level transmitter or a 0–10 V temperature signal.
Typical outputs include a contactor coil, warning lamp and solenoid valve. Analog outputs can command a control valve position or the speed reference of a variable-frequency drive. Modern PLCs also exchange data with drives, remote I/O, HMIs and supervisory systems over industrial networks.
A low-level switch and a high-level switch are the inputs.
A pump contactor is the output.
When the low-level switch turns on, the PLC starts the pump.
The pump remains on until the high-level switch turns on.
An overload contact, emergency stop or maximum-run timer stops the pump and raises an alarm.
That last point is important: the program is not just sensor → motor. It also contains permissives, interlocks, alarms, manual/automatic modes and a defined safe state for faults.
PLCs are used because they provide predictable timing, electrically isolated industrial I/O, easy troubleshooting, modular expansion and maintainable logic. A technician can see which input, condition or interlock is preventing an output without rewiring the control panel. The PLC coordinates the process, but hardwired or safety-rated protection should still handle functions whose failure could injure someone.
A PLC (Programmable Logic Controller) is a rugged computer that repeatedly reads what is happening in a machine, makes decisions using a stored control program, and commands the machine's devices. Unlike an office PC, it is built for electrical noise, vibration, heat and continuous operation.
Its normal scan cycle is:
Typical inputs include a pushbutton, limit switch, photoelectric sensor, pressure switch and emergency-stop status. Analog inputs represent a range rather than only on/off: for example, a 4–20 mA level transmitter or a 0–10 V temperature signal.
Typical outputs include a contactor coil, warning lamp and solenoid valve. Analog outputs can command a control valve position or the speed reference of a variable-frequency drive. Modern PLCs also exchange data with drives, remote I/O, HMIs and supervisory systems over industrial networks.
Simple example: automatically filling a tankSimple example: automatically filling a tank
That last point is important: the program is not just
sensor → motor. It also contains permissives, interlocks, alarms, manual/automatic modes and a defined safe state for faults.PLCs are used because they provide predictable timing, electrically isolated industrial I/O, easy troubleshooting, modular expansion and maintainable logic. A technician can see which input, condition or interlock is preventing an output without rewiring the control panel. The PLC coordinates the process, but hardwired or safety-rated protection should still handle functions whose failure could injure someone.