Block Diagram for a PLC

In this article, we will learn about PLC through block diagrams and a few methods to draw such diagrams. This article will also cover all the important details and working of PLC devices.

In this article
  1. What is a Programmable Logic Controller (PLC)?
  2. What are the Features of PLC?
  3. How to Draw a PLC Block Diagram?
  4. Conclusion

Part 1. What is a Programmable Logic Controller (PLC)?

Technology has made our difficult tasks easier. We are now relying on machinery more than ever. Especially when working in the automation industry, we have to manage our tasks within quick time frames and with less effort. This is possible only through something like PLC. PLC stands for Programmable Logic Controller.

But the question arises: what is PLC, how does it work, and what possible applications could be? There could be more questions related to PLC. In this article, we are going to discuss these questions and learn the basics through a PLC block diagram. This diagram would unleash most of its working and help us know how beneficial these devices could be.

In this guide, we are going to discuss all the facts about the PLC, including the definition, block diagram, working principle and much more.

Block Diagram of Programmable Logic Controller (PLC)

The block diagram for the PLC is as simple as it could be. We need to take a look at a few important sections, and their purpose in the diagram or model needs elaboration.

Here is the block diagram for plc.

plc block diagram

The important parts of a plc are given below.

Processor or CPU

The processor is the brain of the PLC as it contains memory slots, RAM, ROM and a logic solver. The processor works based on given instructions to it. It doesn't follow anything else. The programs are based on a user manual installed in it. It is known as the sensitive part of the plc.

The memory lines that store information are attached to the CPU. The memory section reads the information from the input and stores it in a specified place. It also stores data that is given by the user manual. The output signal is stored in this section and then displayed on the screen.

Input/Output Modules

The input and output modules are similar but receive and send signals differently. The input is a mediator between the CPU and input devices. The analogue signal is converted into a digital signal through the input module, and after processing through the CPU, the electric signal is then changed into an analogue signal and shown on the output devices connected to plc.

Programming device

All the PLC operations work collectively well due to this programming device. When a signal is received through the inputs, this programming device attached to the CPU offers its programming to the CPU to work with. This device is already loaded with the information and phases through with the received signal that has to pass. Without this, a PLC is not a PLC.

Power Supply

The processor unit and all other units are provided with the power supply. Usually, a PLC runs at 0 volts DC and 24 Volts. Without this power Supply, the system doesn't start working.

Part 2. What are the Features of PLC?

Here are a few of the features or characteristics of a PLC.

  • PLC could be programmed in different languages.
  • It receives the same type of orders and executes them in the same fashion for a longer period without fluctuations.
  • It could be connected to a supervisory team that checks and keeps an eye on the machine's actions. This team could even give new orders to perform in the machine.
  • There is the distribution of sensors which act differently to a signal received in the machine.
  • In real-time, the PLC communicates with different teams as well.
  • The interface of the PLC is simple, and the machine operator finds it easy to start things right away.

PLC Working Principle

The working principle for a PLC is simple. The inputs and outputs have all the roles. The next thing needed is a user-created program on what a PLC runs. The input is received through sensors, and the Ignition point could be a button or automatic sensing waves.

A PLC program runs in cycles. The state of the input is crucial and is detected by the plc. The input is changed, processed through the plc, and changed into desired outputs.

working principle of plc

Output signals are altered and sent to the corresponding device connected to a PLC. This process is repeated repeatedly, and plc checks if everything is in the right place. This repeatable property of a PLC makes this device more effective.

The output signals are just like the input signals, but they are more visual and audible, serving the purpose of the PLC machine. The indicators could be the opening/closing of a valve, heating sensors, or the speed of the motor. All these alerts are collected with the help of a PLC machine.

Part 3. How to Draw a PLC Block Diagram?

There are two methods to draw a PLC block diagram. Let's take a look at those two methods.

Method 1: Starting from Scratch

This method is more of a conventional way to design something. In earlier days, when technology was not advanced, this method was used to draw diagrams on paper. These papers are of good value when learning a machine system or repairing it. If an issue arises in the system, you must find this piece of paper to see the design. You can draw these designs on your laptop by drawing every diagram component.

Open EdrawMax, select a new project and select a blank drawing or block diagram.

open EdawMAx and select new file

Collect all the block diagram components or draw them one by one on the canvas.

add shapes to the canvas

Label all the blocks for better understanding and learning purposes.

add connections and label the blocks

Connect each block or component to the other in the right manner and double-check if they are placed correctly.

add colors and connections

Your block diagram for PLC is ready to show.

Method 2: Starting with a Template

It is good to get things ready quickly. Isn't it? Yes, while working on your computer, you always want to complete a given task quickly. Computers are made for your ease and take a load of your work. So, you need to use all the resources available. A template is one kind of thing that saves you a lot of time. Even if you're not a professional designer, you still would be able to complete a diagram with the help of a template. Here are a few steps you need to follow while using a template.

Step 1: Search for a suitable template to start with.

search for templates

Step 2: Choose from the given options.

pick a template for you

Step 3: Download this template and open it. If you want to create any other design rather than a PLC block diagram, you could also choose different options. Just write a diagram name in the search bar and find the best-suited template for your design.

download the template and open it

Step 4: Start with deleting unwanted elements or components in the diagram. Some elements could have nothing to do with your diagram or design. Just click on those elements and delete them one by one.

delete unwanted items

Step 3: Now start labelling the blocks according to your needs. You can even copy blocks and paste them on the canvas if you need more of these blocks for your model. It's super easy. After labelling the block, you can give colours to the blocks if necessary.

add labels and connections

Add background to your design if it is needed. It will make your design look better and save the file if you need any changes at any point in time.


PLC is quite helpful at industrial levels. However, they could be used for personal uses as well. This guide taught us about the PLC through the PLC block diagram. The PLC diagram shows the working process of the machine and helps us at the industrial level.

This article taught us the working principle, the features, and how to draw these diagrams. Plc. has greater value and offers a lot of convenience for us. So, if you want to design models like a PLC block diagram, you need a professional touch. A template could provide such a touch and offer a great design that helps.

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Edraw Team
Edraw Team Apr 15, 24
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