09 Difference between Compact PLC and Modular PLC | Block Diagram

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When I started learning Programmable Logic Controller PLC programming, I found it very difficult to understand the actual difference between compact PLC and modular PLC.

So, to make this simple for everyone, I am writing this tutorial with detail explanations.

As we all know, the programmable logic controller (PLC) (hardware) has two main categories as follows.

  1. Compact PLC
  2. Modular PLC

Now, let’s compare two PLCs.

Difference between Compact PLC and Modular PLC

#ContentCompact PLCModular PLC
01NameCompact PLC is also known as Fixed PLC.Modular PLC is also known as Rack-Mounted PLC.
02I/OIn Compact PLC, the number of inputs and outputs are fixed.  

Because I/O capabilities are decided by the manufacturer but not by the user.
In Modular PLC, the number of input and output are not fixed.  

Inputs and outputs can be added to the modular PLC systems by the user.
03ModuleIt has inputs and outputs modular fitted with CPU. In this PLC,  several components are fitted on chassis or rack or bus with different slots.
04RepairCompact PLC is not easily repaired.Modular PLC is easy to maintain and repair as compared to compact PLC.
05MemoryIt has fewer memory capabilities to store data/information. It has more memory and capabilities to store more data.
06UseIt is useful for smaller applications and most suitable for domestic purposes.It is used for industrial purposes and also for future industrial expansion and growth.
07SizeIt is smaller in size.It occurs in a large size with i/o connectivity, power supply, computing capabilities, etc.
08CostIt is an economic model.It is costlier than Compact PLC.

09. Block Diagram

Compact PLC block diagram: It includes input, output along with power supply and CPU.

Block diagram of compact plc

Modular PLC block diagram: It consists of Power Supply, CPU, and Input/Output Modules, Interface Module.


In the modular PLC, one rack consists of nine or eighteen slots.

Modular PLC Rack with slots

Out of 9th or 11th slots,

  • the power supply connects with the slot number-one,
  • the central processing unit (CPU) is fixed in slot number two and
  • the interface module is fixed in slot number three.

Remaining slot numbers fourth to eleventh is used for the input and output modules, signal module, a communication module, an interface module, a peripheral module, a function module, etc.

Mostly, modular type PLC is used in automation industries. The more different brands of modular PLC are available in the market with their manufactured brand.

Hopefully, this difference between compact PLC and modular PLC gives you a better understanding. If you have any point to discuss, feel free to write in the comment.

If you are interested in learning PLC programming, here is PLC programming related tutorial from beginner to end.

It is simple to start with…

Ready for Test:

If you are ready for online test, click on the PLC Automation Quiz.

Happy PLC Programming!

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34 thoughts on “09 Difference between Compact PLC and Modular PLC | Block Diagram”

  1. In the official site of OMRON I saw 3 categories Compact PLC, Modular PLC and Rack PLC……can u tell me whats the difference between Modular PLC and Rack PLC

    Btw thanks for sharing this !

    • The Rack PLC is the mounting base plate of the Power supply, CPU, Interface module and I/O modules. I think, there is not much difference between them.

      I didn’t get the chance to work on Rack PLC.

    • Hi, Kacung Jongos.

      Yeah,Sure! In the before tutorial ([Step-by-Step] How to Learn PLC Programming at Home for Free?), I shared the PLC software link of free basic version for practicing and learning PLC programming.

      If you are new and learning PLC, I guess using those free version of any PLC software brand is sufficient for your need.

      You can download Delta PLC software from its official website for free. Choose the version suitable based on your computer system. Install it.

      You can also download WPLSoft PLC software and install it. I am working on this PLC Software.

  2. I am a new plc learner. Loved the explanation. I wanted help from your end in the project where I am stuck. Can it be discussed further for a solution as required at present for the progress of it?

  3. Waoooh، like seriously, you are great woman! I wish I could be your successful trainee on PLC programming (software skill).
    Thanks for this and I pray that your knowledge will never depreciate

  4. Dear madam,
    Firstly I want to say thank you to you because you did explain it so easily formats. Ma’am, I want learn more about electronic devices. Can you teach me personally, If it possible so.


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