PLC Working Principle

Function of PLC

Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.

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

While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:


PHASE 1 – Input Status scan

  • A PLC scan cycle begins with the CPU reading the status of its inputs.

PHASE 2– Logic Solve/Program Execution

  • The application program is executed using the status of the inputs

PHASE 3– Logic Solve/Program Execution

  • Once the program is executed, the CPU performs diagnostics and communication tasks

PHASE 4 - Output Status Scan

  • An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.

As soon as Phase 4 are completed, the entire cycle begins again with Phase 1 input scan.

The time it takes to implement a scan cycle is called SCAN TIME. The scan time composed of the program scan time, which is the time required for solving the control program, and the I/O update time, or time required to read inputs and update outputs.

The program scan time generally depends on the amount of memory taken by the control program and type of instructions used in the program. The time to make a single scan can vary from 1 ms to 100 ms.

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A Programmable Logic Controller (PLC) is a digital computer that is designed to control industrial processes and automate machinery.

PLC operates based on a specific working principle that involves the following steps:

Input Stage

The PLC receives signals from various input devices such as sensors, switches, and transmitters. These devices provide information about the current state of the system or process being controlled, such as temperature, pressure, level, or position.

Scan Cycle

The PLC operates on a continuous scan cycle, which is the basic unit of its functioning. During each scan cycle, the PLC performs a series of tasks in a sequential manner.

Program Execution

The PLC executes a user-defined program that is stored in its memory. This program is typically created using a specialized programming language, such as ladder logic, function block diagram (FBD), or structured text.

Input Scan

The PLC reads the status of input devices connected to it. It detects any changes in the input signals since the previous scan cycle and updates the corresponding memory locations with the new values.

Program Evaluation

The PLC evaluates the program logic based on the current values of input signals and the stored program. It determines the appropriate actions to be taken based on the defined conditions and instructions.

Output Generation

After evaluating the program logic, the PLC generates output signals that control various output devices such as motors, valves, actuators, and displays. These output devices perform specific actions to control the industrial process or machinery.

Output Scan

The PLC updates the output signals based on the instructions generated during the program evaluation. It sends the updated signals to the respective output devices.

Repeat Cycle

The PLC continues to repeat the scan cycle at a predefined scan rate, typically in the range of milliseconds. This allows the PLC to continually monitor and control the process or machinery in real-time.

By following this working principle, PLCs provide precise and reliable control over industrial processes, offering benefits such as flexibility, ease of programming, and scalability. They are widely used in various industries, including manufacturing, energy, transportation, and automation.