The working principle of a PLC (Programmable Logic Controller) is based on continuously reading input signals, executing the program logic, and controlling output devices. It works like a loop — first, the PLC takes input data from sensors, switches, or push buttons. Then it processes this data according to the program written by the user. Finally, it sends output signals to operate devices like motors, solenoids, or lights. This entire process repeats continuously, many times per second.
This continuous loop is called the PLC scan cycle, which follows three main steps: Read – Execute – Write. In the Read phase, the PLC checks the current status of all inputs. In Execute, it runs the logic written in the Ladder Diagram or Function Block program. In Write, it updates the outputs based on the result. Because this cycle happens so fast, the PLC can respond instantly to any change in the system. In short, a PLC works by constantly monitoring inputs, processing logic, and controlling outputs to keep industrial automation running efficiently.