We have to start by saying that PLC systems can be categorized in following: processor, inputs,and outputs. These three components are working together by taking inputs, performing logic on them and activating or deactivating outputs based on the input controls and status as well as logic.

The brain of PLC is the processor or CPU. We can easily compare it with the humanbrain, but of course, it is not as complicated as the minds are. We are going to give you an example of a human brain so that you can understand the main idea of CPU for PLCs.

It is the home for memory, logic,and communication between inputs and outputs. Similarly, the human brain is the primary source for memory, decisions,andcommunications that we make to the other parts of our bodies.The program that you will get for the PLC is stored in the CPU.You should check this website: https://onlineplcsupport.com so that you can learn more about ladder logic and how to use it for your specific requirements.

The developer has created a customized program specifically made for your industry and machines in the processing facility. The programmed logic tends to detect various input stated and to activate or deactivate output actions based on the input effectiveness.

How Does CPU For PLC Function?

If we use the comparison to the brain, you should know that it takes input signals such as sound, taste, smell, feel, sight and produces output actions such as talking, gripping, reaching, stepping based on the programming that we have in our brains.

However, this is the place when the analogy doesn’t work anymore because the programming or logic in our brains is not fixed based on the program, but it is adapting to new teachings and experience.

In the past, PLCs used only Ladder Logic as the programming language. However, some new and advanced PLCs will allow you to use other languagesas well such as Function Block Diagram, Sequential Function Chart, Structured Text,and Instruction List.

  • Memory – The memory is contained in the CPU,and it is the place where all programs and data are stored both permanently and temporarily. It is similar to ROM and RAM of the PC. If we compare it with a human brain that stores data permanently and temporarily, so that we can conduct and achieve specific tasks such as driving the car, which we use from memory.
  • Communications –Apart from storing programs and memory within it, the CPU also handles the communication. It is done by USB port or serial on CPU module by connecting the computer that the programmer is using. The connection can also include input and output modules by using chassis backplane. You can also communicate with other industrial automation devices by connecting it with Ethernet or different network types.

If we continue using the analogy with the brain, the communications we’ve mentioned above can be compared with communication with teacher, experience and some force that is above us, to the brain, and brain communication to other, other body parts and finally, the communication between two brains.

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The logic of our brain goes like this: it receives inputs from our eyes, for instance, the person we see who is on our way. Then the brain creates a logical decision that you should move to avoid a collision. The brain will tell legs to bend and your body to move so that you can finish what you wanted.

Of course, you have to know that brain is much more complex machinery and programming than the one we are explaining, and it is also more flexible, adaptable and powerful than PLC, but it is a great way to understand the similarities between the human control system and PLCs.

That way you will be able to understand the correlation between PLC brain and the human brain, and you will notice how you can easily program PLC to perform numerous tasks that we would usually do manually.

What Is Ladder Logic?

Ladder Logic is the intuitive programming language that was meant to replace hardwired relay logic that we used before for automation industries. When you place relay logic on drawings, you will get ladder diagram.

We have noted above that today’s programmable logic controllers are using other languages too, but Logic is still the most popular and convenient especially because it is made explicitly for PLCs.

One of the significantadvantages of Ladder Logic is the ability to troubleshoot it in a matter of minutes. Since it is visually-based language, it is convenient and straightforward to spot when the circuit is stock.

At the same time, since it’s similar to relay control diagrams that will allow technicians, engineers,and electricians the ability to transition with ease between relay logic and programmed Ladder Logic.

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Basic Instructions

You can find thousands of guidelines and tutorials that will explain to you how Ladder Logic functions, you can install a software package that will allow you to program with ease. There are a few things that you will need to know as a PLC programmer.

  • Relay-Type Instructions –The most important instruction is relay instruction, and the most common ones are XIC (examine if closed), OTE (output energized) and XIO (examine if open). These three instructions can be compared with open contact, normally closed contact and relay coil.
  • Timers and Counters – The next most common instructions are counters and timers. These instructions will count and represent the time for the specific process. The instruction which is mostly used is TON (timer on delay). This particular instruction starts when the rung becomes active. The idea is to setup the preset value and to set DN so that you can trigger side of the logic.


Programming for PLC is convenient especially if you have knowledge that will help you understand the relays and logic, as well as an electronic circuit.

Since it is visual language, you will be able to see what is wrong and to fix it in a matter of minutes, which is the main reason for its popularity