Site Loader

They are considered as one if not the best, programming languages are Programmable Logic Controller language. It is called a ladder logic or diagram, and people can learn the system very fast if they put their mind into it. The best thing about the diagram (LD) is that it is the same with electrical relay circuits.

That is why, if people already know and understand a little bit about electrical circuits and relay controls, they can learn the ladder diagram even faster. In this article, users will learn everything they want and need to know about LD Programmable Controller language. People will be able to start operating real PLC programs using logic in any PLC software.

What is this diagram?

It is a PLC programming language, also known as LD. It is also what we will call in this article. There is a simple reason for the name. LL or LD is made of rungs that looks like a conventional ladder. It is not impossible to scale a PLC input for instance, although LL is usually for logic operations.

Even simple logic operations can be useful in a more advanced PLC program and SCADA or Supervisory Control and Data Acquisition system programming. Organizations or individuals that set the standard for LL is PLCOpen. The language is not only an excellent programming language for controllers.

Visit for more information about Supervisory Control and Data Acquisition system programming.

It is a standardized programming language used for PLC systems. It means that LL is described to a degree. That degree is called IEC 61131-3. But at this point, the only thing beginners need to know is that there’s a degree or standard that describes the programming language.

Introduction to LL or LD

To get started with this language, there are few things people need to know about programming languages. People need to know why LL was invented. That way, it will be a lot easier for users to understand it, especially if they have previous experiences with electrical relays and circuits or experience dealing with Boolean logic.

The system was invented for experts and technicians

LD is a graphic-laced programming language. It means that instead of using texts, the system uses a combination of various graphic elements. These components are called symbols. One of the best things about these symbols is, they made to look like regular electrical symbols.

This LL is initially designed for electricians, technicians, as well as people with a background in electrical systems. People who know how to look at electrical diagrams and schematics are the target of this system. Just like in electrical diagrams, these LDs have symbols for relays and contacts (it is called coil in LL). These symbols might look different from the symbols people find in a lot of electrical schematics, but these symbols have almost the same purpose or functions.

How to read LL and LD?

Another difference between these diagrams and typical electrical diagrams is the way the two are drawn. While electrical schematics are usually drawn horizontally, LL diagrams are designed vertically

Easier to read

First and foremost, it makes LD easier to read since it’s natural for the user’s eye to go from right to left and vice versa, as well as going down to the next line – just like when people read a book. Of course, it applies only to people that are living in places or countries where reading from left to right.

Drawn on Personal Computers

When technicians draw LL on computers, they will make one rung at a time. As they draw more rungs (or lines in ladder logic), they will start to stack upon each other, making these lines look like ladders. The best way to look at these diagrams with a lot of lines is to scroll along with the screen vertically.

Order of execution

Another reason to draw LL vertically is to set order orders of executions. It is how the Programmable Logic Controller will run the system. To be accurate in what the Programmable Logic Controller will appropriately execute the order of the ladder diagram instructions. PLCs will always start at the top of the ladder and complete the process going down

Relay ladder logic

AS we said before, LDs can look like electrical schematics that go vertical. A lot of people learn to draw the diagram this way – by building electrical schematics. But there are differences. It is why experts tell users to learn it in different ways. The problem is that most electrical systems and PLCs work in different ways. Listed below are their most prominent differences.

The Programmable Controller takes one line or rung and executes the rung and then goes on the next line.

In some electrical systems, current pathways or multiple lines can be activated or executed at the same time.

With the crucial differences mentioned above in mind, let us get into it. It is time to learn the basics of LL.

Ladder Logic basics

The first thing people will see when they design a new piece of the diagram are two vertical lines. It’s in between these lines where the system will go. When technicians draw LL, they will draw some vertical lines to connect the two limes. The lines that were drawn are called rungs – just like the traditional ladder.

In these lines, people can put different LL symbols to help create the logic that they needed to run the system. It is to help understand how PLCs will execute the diagram. People may be familiar with PLC scan times of scan cycles. Roughly speaking, the Programmable Controller will first scan all the inputs, and execute the written program to help set the outputs.

But how do these PLCs implement the LL? They perform it one rung at a time. It might be one, if not the most relevant rules of LL. PLCs only run one rung or one at a time, then go on to the next one after. As a matter of fact, Programmable Controllers only run one line and one symbol at a time.


  • Partner links