[Explained] PLC Block Diagram | Programmable Logic Controller

Hey, today we are going to see and discuss the basic PLC Block Diagram. You will easily understand the working principle of PLC from the block diagram of PLC. PLC stands for Programmable Logic Controller. PLC is a very popular technology in the industrial sector for automation purposes. PLC replaced the analog control system by bringing the digital control system into the industry. It is a specialized computer system used in industrial and manufacturing settings to automate and control various processes. PLCs are highly flexible and can be easily reprogrammed or modified to accommodate changes in the industrial process or requirements.

Programmable Logic Controller(PLC) Block Diagram

Here a typical block diagram of PLC is given below. From the below block diagram, you can understand the total concept and working procedure of the Programmable Logic Controller.

PLC Block Diagram, Block Diagram of PLC, Programmable Logic Controller

The above block diagram illustrates the connection between the input modules and the input devices (sensors, switches, etc.) as well as the connection between the output modules and the output devices (actuators, motors, etc.). This helps visualize how the PLC system interacts with the external world.

This block diagram also shows the central processing unit (CPU) of the PLC system and its connection to the various modules. It demonstrates how the CPU receives input signals, processes them based on the programmed logic, and again sends the generated output signals to the output devices through the output module. This highlights the core functionality of the PLC system.

Main Important Parts of a PLC System

As you saw in the above block diagram the PLC has the following important parts,
  1. Input Module
  2. Output Module
  3. Central Processing Unit(CPU)
  4. Memory
  5. Power Supply Module

Generally, a microcontroller or microprocessor is used as Central Processing Unit in a Programmable Logic Controller and they are programmed by an external computer. After installing the appropriate program, PLC works.

PLC Working Principle

Now, let's discuss each block of the Programmable Logic Control circuits which will help you to understand the working principle of PLC.

CPU(Central Processing Unit)

It is the main part of the PLC. It processes all the instructions required for the working of the circuit. First, we store the program or instruction in memory. In the operation time, the CPU takes the commands from the input module and then process and ultimately gives the output to the output module. The common functions of the CPU in a PLC system are Program Execution, Data Processing, Communication, Memory Management, Diagnostics and Fault Handling, Real-Time Control, Timing, Synchronization, Security and Access Control, etc


It is a storage device that stores all data, programs, and instructions. There are different types of memory in a PLC system such as Program Memory(it stores the PLC's operating system, firmware, and the user-defined PLC program or logic), Data Memory(it stores variables, counters, timers, and other data required for the PLC program's execution), I/O Memory( It is used to exchange data between the PLC and the connected input and output modules). Some PLC systems offer the ability to expand memory capacity by using external memory modules or memory cards. These external memory modules provide additional storage for programs, data, and configuration information. This helps the PLC system to handle larger and more complex applications.

Programming Device 

It is the device where the program or instruction is written and then using this device the program or instruction is stored in the PLC Memory. The programming device may be a computer, laptop, etc. PLC programming is performed using specialized software provided by the PLC manufacturer. The PLC programming software is designed to run on a computer operating system, such as Windows. The software provides an interface for creating, editing, and organizing the PLC program, configuring I/O modules, setting communication parameters, and performing other programming-related tasks. So you need to run the software on your computer and it will allow you to create, edit, and transfer the PLC program to the PLC hardware for execution.

Input and Output Module

The CPU or Microprocessor can work with a 5V DC supply and it can deliver a very small output current. But the input devices or sensor may not work with the same voltage that is 5V DC. So to interface real-world input devices and sensors with the microprocessor, the input module is used. Here the input module always gives a 5V DC input signal to the CPU. 

Another important point is that the microprocessor can work with only the digital signal but all the input devices or sensors may not produce a digital signal. In fact, most of the sensors create an analog signal. So another important function of the input module is to convert the analog signal into a digital signal.

The Microprocessor or CPU can deliver a very small amount of current(in a few mA) at 5V DC as output. So we can not drive the loads directly with the microprocessor. Here the output Module solved this problem.

Power Supply

The power supply unit provides the power supply to all devices such as the CPU, Memory, Input module, and output module. PLC power supplies typically operate within specific voltage ranges, such as 24V DC, 120V AC, or 240V AC, depending on the system's design and application. When the PLC uses 24V DC, you need a 24V DC power converter to operate the PLC.


I think you may easily understand the working principle of a typical PLC system with the above block diagram. There are many advantages of PLC or Programmable Logic Controller which is why it is widely used. The main advantages of PLC are

1.      Industrial automation using PLC is very efficient.
2.      They are very fast in operation.
3.      Additional advanced tasks can be performed using PLC.
4.      Easy to use and very good experience.
5.      Automated control is also a great advantage of PLC

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[Explained] PLC Block Diagram | Programmable Logic Controller [Explained] PLC Block Diagram | Programmable Logic Controller Reviewed by Author on June 03, 2019 Rating: 5
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