PLC Ladder Logic Tutorial

PLC or Programmable Logic Controller is considered as one of the best programming languages in the world today. It is called a ladder diagram or ladder software, and it is very simple, most companies can learn it very fast. The best thing about it is it has the same concept as an electrical relay circuit.

So, if you have an idea on how to use electrical circuits and relay control, you will have an easy time learning the software. In this tutorial, people will learn all about the software and everything they need to know about diagram programmable logic controller programming language. After reading this article, you will have an idea on how to make a programmable logic controller program with ladder software in any PLC software.

What is a Ladder Logic?

Ladder software is a programmable logic controller programming language. It is called LD or ladder diagram, but most programmers are used to calling it as ladder software. In this tutorial, that’s what we will call it. It is called ladder software because it is made of rungs that make it looks like an actual ladder.

The software is for bit logic operations, but it is also possible to scale a programmable logic controller analogue input. LL or ladder logic is not only a programming language used for PLC’s, but it is also one of the standardized programming languages used in the programmable logic controller.

It means that LL is described as a standard called IEC 61131-3. But right now, all you need to know is that there is a standardized programming language used in PLC’s.

Ladder logic introduction

There are a few things you need to know about programming language if you want to start with LL or ladder logic. To know what the software is, you need to understand why it was invented in the first place because then it will be easier to understand the process behind it.

It is also essential to know its history because it will give you an idea of how it works mainly if you have past experiences dealing with Boolean logic, relays and electrical circuits.

Ladder software is invented for technicians

Ladder logic or LL is a graphic language used in programming, which means, instead of using text, the programming is done using different kinds of graphic elements. These elements are called, in the programming world as symbols. The best thing about using the software’s symbols is that they look like standard electrical symbols.

Ladder software was made for electricians, technicians and people with knowledge of electrical circuitry and people who know electrical schematics and diagrams. Although the software’s diagram has different symbols for relays and contacts compared to the electrical diagram and schematics, it is essential to know that they have almost the same functions. If you want to know more about Boolean logic? Click here.

Reading ladder logic

Another difference between the electrical diagram and ladder software schematics is the way how they are drawn. Electrical diagrams are drawn horizontally, while the software’s schematics are usually drawn vertically.

Easier to read

Because the software is drawn vertically, it is much easier to read because it has natural for the eye since it is drawn from left to right and down to the next line. It is easier for people living in countries where reading is done, left to right.

They are usually drawn on a computer

When you are drawing the software using a computer, you need to make one line at a time. As you draw more lines (or rungs in ladder logic), the lines will stack on top of each other, making it looks like an actual ladder. The best way to look at a big ladder schematic with a lot of lines is to scroll vertically on the computer screen.

Order of execution

The last reason in drawing ladder software horizontally is that it will set the order for execution. PLC or programmable logic controller will run your ladder using this order of execution. To be more accurate on what order of execution your ladder software is instructed to do by your PLC, the PLC should always start at the ladder’s top and execute its command, way down.

Relay Ladder Logic

As mentioned above, ladder diagram and a vertical electrical schematic look remarkably the same. Most technicians and electricians learn how to draw the software’s schematics by building electrical diagrams. But there’s always a difference.

That’s the reason why it is advisable to learn both symbols to make sure that you will know the difference between the two. One problem is that PLC and electrical control system work differently.

PLC or programmable logic controller takes one ladder line or rung and executes it then goes to the next line. With the electrical system, multiple lines are performed or activated, all at the same time.

(To know more about ladder logic, visit

Ladder Logic Basics

The first thing you need to see when you are creating a new ladder logic is the two vertical lines. It is in these two vertical lines where the software will go. When you are drawing a ladder software, you need to draw a vertical connection between the two lines.

Those lines are called a rung, just like in an actual ladder. With these rungs, you can put all the ladder software signs to create the system that you want. If you are familiar with the programmable logic controller scanning cycle or scan time, you will know that the PLC will scan all the inputs and execute it to set a programmable output.

The Programmable logic controller will implement the ladder software one line or rung at a time. It is one of the most critical rules in ladder logic. The programmable logic controller will only execute one line or rung at a time and will go to the next line after it is finished with the first one. A programmable logic controller will only execute one symbol at a time — nothing more, nothing less.

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