It is essential to state that PLCs or programmable logic controllers are small industrial computers that feature modular components so that you can control the automation processes and industrial settings.

In most cases, PLCs are popular for industrial plants and factories to control lights, pumps, motors, circuit breakers, and other analog and digital machinery. The best way to understand their overall purpose is by understanding how everything started.

Check out this link: to learn everything about ladder logic.

Have in mind that industrial automation started way before programmable logic controllers appeared on the market. Since automation requires complicated electromechanical relays, the process was challenging for the basic automation system.

The amount of wires relays and space for entire automation was problematic, especially since a thousand relays were necessary to handle a small process at the factory.

At the same time, when the troubleshooting happened or when someone wanted to replace relays, the downtime was significant, which lead to severe production expenses.

That was the main reason why the first PLC entered the market with an idea to replace problematic relay circuitry by implementing a microprocessor as the alternative.

It was a convenient alternative because it used similar programming as relays so that both engineers and technicians could handle it without additional training.

The first ones were used by ladder logic programming that resembled control circuit schematics, and that is when the ladder diagram entered the market with an idea to create a power flow that goes from left to right.

They were the same as the control circuit’s schematics; however, the input sources included proximity sensors, pushbuttons, and switches, among other things. You can see the inputs on the left, while the output sources were on the right side of the scheme.

The idea was to program the complex processes by using the intuitive interface as well as a programming language that used similar graphical representation as relay logic beforehand.

Even though they had limited speed and memory characteristics, the years improved them to a point where we can find a simplified design that could handle any task that you want to implement within the production line.

How Do They Operate?

We can easily say that programmable logic controllers are small industrial computers that feature modular components that are controlling most industrial automation processes.

At the same time, they feature these components:

  • CPU or processor is the brain of the controller
  • Inputs
  • Outputs

It is vital to state that they are both powerful and sophisticated computers that feature various functions that will help you handle the automated processes.

The idea is that they use inputs, perform particular logic depending on the programming you implemented, and turns the proper output based on what you wish to achieve.

Analog and Discrete Inputs and Outputs

Often when you see the abbreviation such as I/O, we are talking about inputs and outputs. They can be digital or analog, depending on the type of process you wish to handle. Discrete signals are the ones that can only be on or off.

Remember that they are the most common type of input/output perspective. On the other hand, you can also use analog inputs and outputs within a particular system.

The idea is to learn more about PLC ladder logic so that you can become a professional and improve your overall career as time goes by.

By using the analog signals, instead of the on/off as the only possibilities, you can measure each input and output and set it based on the preferences you wish to program beforehand.


Have in mind that the programmable automation controller is another form of modern or advanced PLC that entered the market back in 2001.

Even though it differentiates from the original controller by using the latest, more flexible, and more powerful components, it still comes with similar procedures that are only taken to the next level.

We can provide you numerous differences between these two controllers; however, in reality, most industries are interchanging them depending on the process they wish to handle.

The idea is to handle the particular situation and automation process by reducing the overall expenses and minimize the costs. Even though PACs can handle more significant and sophisticated automation systems, if you are using the basic ones, you do not need it in the first place.

Let us see the most common differences and similarities you should understand:

  • Usage – Have in mind that PLCs feature built-in networks that will allow them to communicate with other controllers as well as supervisory-control and data acquisition systems. On the other hand, PACs feature modular design as well as open architecture so that you can control, monitor, and communicate with multiple devices and equipment depending on what you need to achieve.
  • Memory – PACs tend to incorporate simplified programs, which is why they have a vast memory size so that you can handle large systems and projects. This particular feature will allow you to eliminate particular components and include tag-based programming. PLCs also feature high execution speed but limited memory and ability to separate both input and output devices. Of course, the advanced models will come with a high input/output sequence as well as the ability to control analog I/O and PIDs, among other things.
  • Programming – You can use ladder logic programming, which is available graphical language so that you can handle the automation processes with ease. PLCs are convenient, but PACs come with other options because you can use other languages as well, such as function block diagrams, structured text, and other PC programming languages such as C++ or C.