PLC Training with Online Resources: PLC CPU or Processors

PLC Training with Online Resources: PLC CPU or Processors

In the previous article, we talk about the history and introduction to the world of Programmable Logic Controller and how it works in the industrial setting. After reading that, you are now ready to start with this article, where we will take a closer look at the programmable controller’s Central Processing Unit or the CPU by using our brain as an analogy. We will also talk about some of the examples of Allen-Bradley PAC and PLC processors.

Note: In the world of industrial automation, it is prevalent to use the term CPU, controller or processor interchangeably. That is why you don’t need to get confused when this article does that.

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What is a Programmable Logic Controller processor?

You need to remember that most of the components of a Programmable Logic Controller system can be grouped into three categories:

Central Processing Unit or processor



We already discussed how these three works together to take the input, performs the logic on these inputs and activate or deactivate the output based on the combination on the status of the data and the programming logic. In this article, we are closing in on the central processing unit memory, where the Programmable Logic Controller logic and memory are found.

Human brain and PLC

The central processing unit is considered as the brain of the controller. It can be compared to our brain, although it is not as complicated as the human mind. That is why we are going to use the mind to help us understand the purpose of the processor a lot better. It is the home of the PLC logic, communications and memory. Just like the processor, our brain is the source of a person’s logical decision, communications and memory going to the other parts of the body and other human minds.


The Central Processing Unit is where this technology program, usually programmed using ladder logic, is stored. The customized programs were made or created by developers to automate any machine used in the industrial plant, processing facilities and factories.

In the dishwasher example, we looked at how a logic program might look at the dishwasher central processing unit. This programmed LL can detect different input states and activate or deactivate different output actions. Programmed logics that are in every PLC processor is the same as our dishwasher example.

However, it will still depend on the application that was used. The processor can also be compared to a human brain in a way our mind takes all the input signals provided by both inside and outside factors like feel, taste, sight, sound and smell, while producing output actions like reaching, talking, stepping or gripping based on our brain’s so-called “programming.” Obviously, the brain analogy breaks down sooner or later because the programming or the logic in our mind is not fixed and adapting to our teaching and new experiences regularly.

You need to remember that traditionally, this technology has been programmed using LL. A lot of these newer PACs or programmable automation controllers will also allow you to use other languages like Structured Text, Sequential Function Chart, Instruction List and Function Block Diagram for its programming.

Visit this site to find out more about SFC or Sequential Function Chart



The CPU memory is usually in the processor and is where the programmable logic controller programs and data are being stored permanently or temporarily. It has the same concept with a computer’s memory (read-only memory or random-access memory). Again, we can compare memory to the human brain, which stores essential data permanently or temporarily and make sure that we can achieve different kinds of tasks like riding a bicycle or walking.


Every Programmable Logic Controller can handle some kind of communication. Communications made by the CPU of a PLC system usually includes one or more of the following:

Communications using USB or serial port on the central processing unit module to the programmer’s computer

Communications to the I/O or input and output modules through chassis backplanes

Connections to other programmable controller and other factory or industrial automation devices using Ethernet or other network types

Continuing with our brain analogy, the PLC communication mentioned above can be compared to the connection from the programmer to the mind, communication from the human mind to the different body parts like the hands, nose, legs or eyes and the interaction between other human brains like talking to other people. Let us use an example. The human brain’s logic can be something like this:

The human mind receives the input from our eyes that an object is on the floor and in your way

It makes a logical decision as to why the thing needs to be picked up

It will tell your legs to go down or bend, arms to reach and the hands to pick up the item and move out of your way

Although our brain is more complex, adaptable and powerful compared to this technology, you can see their similarities, a similarity between PLCs and our body’s control system.

Check out for more information about I/O.

Specifically, you need to know the correlation between the PLC processor and our body’s most important organ. Notice how the technology can be programmed to properly work with every mechanical equipment to perform different tasks that would otherwise have to be performed manually by humans.


There is a lot to know, learn and understand about Programmable Learning Controller and sometimes it feels like all the information is too much for your head to comprehend. There will always be more details to learn about, but at this point, you need to have a better understanding of what this technology does, be able to know what our brain can teach us about processors and be familiar with the most popular PAC or PLC processors in the market today. Basic biology teaches us that our mind is considered as the organ that makes all the decisions when it comes to our bodies. It is the same with PLC; the processor helps the machine to decide on what to do.