PLC Operation

A programmable controller, also known as programmable logic controller (PLC), is a device designed to perform sequential logic control in which one event follows another in a prescribed way to complete a task.

An industrial machine may contain pushbuttons, limit switches, timers, interlocks, etc. In the past, these devices had to be hard-wired to perform a specific function, and they could not easily be modified to perform a different task.

With a programmable controller, however, all that is required is to change the controller’s program and possibly some of the connections to the inputs and/or output.

Today, programmable controllers are used for the control and operation of almost any machine, process, or production line. The continued advancement of PLC technology and the partnership of PLCs with computers and other digital devices are providing industry with sophisticated control systems and advanced decision making processes.


A programmable controller is composed of three components as illustrated in the below Figure. These three components are the input/output (I/0) interface system, the central processing unit (CPU), and a programming device.

PLC Operation

Inputs and Outputs

The input/output system forms the interface by which field devices are connected to the controller. The main purpose of the interface is to condition the various signals received from or sent to external field devices.

Incoming signals from sensors such as pushbuttons, limit switches, analog sensors, motor starters, etc. are wired to terminals of the input interfaces.

Devices that will be controlled, like motor starters, solenoid valves, pilot lights, and position valves, are connected to the terminals of the output interfaces.


The CPU section of a PLC has three components: The memory system, the processor, and the system power supply. The memory system stores the program usually in the form of a ladder diagram.

The processor executes the control program stored in the memory system. The system power supply provides all the necessary voltages required for the proper operation of the various CPU sections.

Programming Devices

The programming device allows the user to enter, change, or monitor a PLC program. The programming device may be a CRT terminal, a hand-held unit with a display or a personal computer.

Four major languages can be used in programming the modern PLC. These languages may use Boolean algebra equations, mnemonic commands, logic diagrams and ladder diagrams.

Some PLCs also permit BASIC language statements or allow the programmer to use a special high-level control language. Ladder diagram logic, which is the same as that for relay control circuits, is a popular choice for programming most PLCs.


During the operation, the CPU reads or accepts the input data or status of the field devices via the input interfaces, executes the control program stored in the memory system, and writes or updates the output devices via the output interfaces.

This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.

Author: Dr. Hadi Saadat

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