Difference Between Compact PLC and Modular PLC

Difference between Modular and Compact PLC (1)

Today, almost everyone is looking for new and unique ways to upscale their industrial processes, considering the increasing rate of competitiveness in the corporate world. Programmable logic controllers have emerged as handy tools for automating tasks. They take away the burden of certain repetitive tasks, allowing you to allocate your human resources for other, more complex tasks.

However, PLC programming is done in various types, and there is often a debate between the most ideal one. This article is also going to discuss on a similar topic. It will deal with the dynamics of Compact and Modular PLCs. So, read on till the end!

What is PLC?

A Programmable Logic Controller (PLC) is a specialized computer used in industrial automation to control and monitor processes. PLCs are designed to withstand harsh industrial environments and are programmed to execute specific tasks. These tasks can be turning machinery on/off, regulating processes, and collecting data.

PLCs use a ladder logic programming language that resembles electrical relay logic. Such a PLC programming makes it easier for engineers to create control sequences. PLCs enhance efficiency, reliability, and safety in industrial applications by providing a centralized control system for various components.

What is Compact PLC?

Compact PLCs are programmable logic controllers that are designed for smaller-scale industrial automation applications with restricted input/output needs. Its small size and simple design make it ideal for jobs where room is limited. Some of the benefits of Compact PLCs are:

  • The small size of compact PLCs saves significant space in control cabinets and makes them appropriate for installations with limited space.

  • These PLCs are less expensive than bigger versions. This cost-effectiveness makes them an inexpensive solution for projects with limited automation requirements.

  • Compact PLCs are user-friendly and simple to connect to current systems. Such ease in connectivity allows for a smooth transition to automated operations with few changes.

What is Modular PLC?

Modular PLC

Modular PLCs are developed to provide flexibility in industrial automation applications. They facilitate the users to add or delete input/output modules. Due to their versatility, they are well-suited for applications with changing automation needs. Some of the benefits of Modular PLCs are:

  • It provides a scalable solution that can adapt to changing automation needs.
  • It simplifies troubleshooting and maintenance, as faulty modules can be replaced without affecting the entire system.
  • It ensures the automation system can evolve to meet future technological or operational demands.
  • Modular PLCs are also called as Rack Mounted PLC, baseline hardware starts with a Rack mount hardware with power supply + CPU unit with different number of slots [3,5,10] available on back panel rack for expansion of various PLC modules selection depending on application needs.
  • Basic hardware design rules state to provide 25% spare slots capacity for future expansion both on number of slots availability and IO points availability on modules on each slot.
  • Programming gets complex with Modular PLC and requires maintainingan actual hardware configuration file for each application which includes the CPU, and specifications model numbers for each module on the CPU back rack.
  • PLC IO scanning times increase with each addition of IO modules, hence scan time optimization is an important design criterion.
  • Power supply selection is critical and power distribution is done via a back panel common bus to IO modules installed on each rack modules.

Difference between Modular and Compact PLC

Despite being PLCs, there are various distinguishing factors between compact and modular PLCs. Some of them are given as follows:

1. I/O Configuration

Compact PLCs have fixed I/O capabilities determined by the manufacturer. This limits its flexibility. In contrast, modular PLCs offer users the freedom to customize and modify I/O configurations by adding or removing modules as needed, providing adaptability for diverse industrial automation requirements.

2. Module Structure

Compact PLCs incorporate inputs and outputs directly with the CPU in a tightly integrated modular format. Conversely, modular PLCs adopt a chassis or rack structure equipped with various slots. This design allows users to insert different modules, providing a more flexible and expandable architecture. Such a module structure is capable of accommodating evolving industrial automation needs efficiently.

3. Repair and Maintenance

Compact PLCs present challenges with intricate repairs. Thus, it often necessitates the replacement of the entire unit for fault resolution. On the other hand, Modular PLCs streamline maintenance processes by enabling the selective replacement of faulty modules. This modular approach significantly reduces downtime and simplifies troubleshooting.

4. Memory Capacity

Compact PLCs have constrained capabilities, potentially limiting data storage. At the same time, Modular PLCs excel with greater memory capacity, allowing for the storage of extensive data and information. This enhanced memory facilitates the handling of larger datasets.

5. Application and Use

Compact PLCs suit smaller-scale and domestic needs, prioritizing simplicity over scalability. In contrast, Modular PLCs find their primary purpose in industrial settings. They provide the necessary versatility for existing operations and future expansions. Their design caters to the dynamic demands of industrial automation.

6. Size

Compact PLCs are smaller, fitting well in environments with limited space. They are designed for efficiency in smaller applications. Whereas Modular PLCs boast a larger size. They accommodate comprehensive I/O connectivity, robust power supply, and advanced computing capabilities.

7. Cost Considerations

Compact PLCs are economical and ideal for budget-conscious applications. On the contrary, Modular PLCs command a higher cost due to their enhanced capabilities. This presents an investment for industries with complex automation needs. The added cost reflects the versatility and adaptability that Modular PLCs offer, making them a strategic choice for sophisticated industrial requirements.

8. Block Diagram

Compact PLCs present an integrated design with input, output, power supply, and CPU effortlessly interwoven. On the other hand, Modular PLCs showcase a modular rack system incorporating Power Supply, CPU, various Input/Output Modules, Interface modules, and specialized modules. This design accommodates diverse industrial automation needs and facilitates future expansions with ease.


To summarize, both Compact and Modular PLC programming are done in different ways to cater to specific industrial requirements. Thus, both have their certain advantages and limitations. This is also why choosing between them depends on your industrial purpose. Compact PLCs are ideal for smaller setups where simplicity and space efficiency are paramount. Modular PLCs, on the other hand, offer scalability and flexibility for larger, more complex automation systems. Understanding the benefits and differences between these two types ensures that you can make informed decisions to optimize your automation processes.

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