What Are the 3 Types of Programmable Logic Controllers?

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What Are the 3 Types of Programmable Logic Controllers?

Programmable logic controllers (PLCs) are a cornerstone of industrial automation, providing the control and flexibility to optimize processes in various industries. While PLCs are often seen as a one-size-fits-all solution, the truth is that several subcategories are specifically designed to meet the unique needs of different industries and applications. 

In this guide, we’ll explore the different types of PLCs, their specific benefits, and how each type can be customized or expanded to meet your operational goals. 

Whether you're upgrading your automation system or starting from scratch, knowing the capabilities of each PLC type is the first step toward making the right decision for your business.

Types of PLCs: Fixed, Modular, and Compact PLCs

There are three primary types of PLCs you need to consider when building or upgrading an automated system:

1. Fixed/Integrated/Compact PLCs

Fixed PLCs, or integrated or compact PLCs, have all their components—the CPU, power supply, and I/O modules—housed in a single unit. This makes them an excellent choice for smaller applications or more confined industrial settings. Fixed PLCs are ideal for facilities that don’t require frequent changes or upgrades to their system, offering a streamlined, space-saving design.

Advantages of Fixed PLCs:

  • Ease of Installation: Because everything is integrated into one unit, installation is straightforward, making it perfect for smaller operations or facilities with limited space.
  • Cost-effective: Fixed PLCs are often more affordable than their modular counterparts, requiring fewer parts to manage and install.
  • Simple Maintenance: Since fewer individual components exist, fixed PLCs require less maintenance and troubleshooting.

When to Use Fixed PLCs:

Fixed PLCs work well in facilities with consistent operational needs and minimal need for future expansion. They are commonly used in small manufacturing setups or remote installations that don’t require frequent changes.

Brands Offering Fixed PLCs:

2. Modular PLCs

Modular PLCs are the ideal solution for industries requiring more flexibility and the ability to scale. Unlike fixed PLCs, modular PLCs allow users to add or remove I/O modules, memory, and communication ports as needed. 


This makes them suitable for larger and more complex applications where production demands change frequently.

Advantages of Modular PLCs:
  • Customizable and Scalable: As your business grows or changes, you can easily add new modules without overhauling the entire system.
  • Greater Processing Power: Modular PLCs can be equipped with high-performance CPUs and handle complex operations in real time, making them suitable for larger industries such as automotive, food processing, and pharmaceuticals.
  • Flexible Communication: Modular PLCs often come equipped with communication interfaces like Ethernet and Profibus, making them suitable for integration into broader industrial networks and advanced supervisory control systems.

When to Use Modular PLCs:

Modular PLCs are the go-to solution for large, dynamic facilities that require adaptability and precision. These systems are often used in industries like automotive manufacturing, where different production lines may need specific controls or where frequent system updates are standard.

Brands Offering Modular PLCs:

  • Siemens offers the highly flexible SIMATIC S7-1500 series, a powerful modular PLC designed for advanced automation tasks.
  • Allen Bradley produces the ControlLogix series, known for its flexibility, scalability, and high-speed processing.

3. Specialized Output PLCs: Relay, Transistor, and Triac PLCs

In addition to the broader categories of Fixed and Modular PLCs, specialized output PLCs cater to specific types of outputs, which are vital to controlling various systems in your facility. 

There are three main types of output PLCs:

Relay Output PLCs

The first type of PLC to consider is the relay output model. This product uses mechanical contacts to control two amps at once. These devices also work very well for systems with very low resistance. Essentially, they simplify connecting your PLC to an exterior device and allow for a more straightforward control method. 

However, they aren’t as well suited for applications requiring quick equipment transitions.

Transistor Output PLCs

Transistor outputs, on the other hand, are solid-state devices designed to work with small or medium current quantities. These PLCs do not have moving components, which makes them less likely to experience damage than relay models. This also makes them more reliable and able to last much longer. 

It’s important to note that transistor outputs only work with DC voltage, so devices cannot be run using AC voltage.

Triac Output PLCs

Another of the three modular programmable logic controllers is the triac output PLC. Unlike transistor PLCs, this type runs exclusively on AC voltage, making it an excellent option for low-power applications. These include controlling lighting, motor starters, and contractors with a single amp rating. 

These devices benefit any output requiring alternating current flows.

Why Choose PLC Direct for Your PLC Needs?

Regardless of the type of programmable logic controller you need for your business, PLC Direct has a vast selection from which you can pick. As a premium PLC supplier, we’re dedicated to providing a wide range of options for several different industry needs.

We offer a wide range of top-quality PLCs from leading brands like Allen Bradley, Siemens, Fanuc, and Omron to meet the specific needs of any industry. Explore our inventory of PLCs today and find the right one to take your automation system to the next level.