Introduction to Ladder Logic With Codesys 3.5

Updated on January 2, 2020
Example Of A Ladder Logic Rung/Network
Example Of A Ladder Logic Rung/Network

About Ladder Logic

Ladder Logic, usually referred to as simply "Ladder", is the most common language used in programming a PLC (Programmable Logic Controller). It has been in use for decades, and the fundamentals haven't changed all that much over time.

The reason it is so popular is because it reads like a schematic, from left to right, top to bottom. It was and still is, relatively easy for electrical engineers to get a copy of the software and read the logic.

In this introduction article, we will be looking at Ladder using CoDeSys 3.5 (which is free, you can find a download link at the CoDeSys 3.5 download center). If you need support with CoDeSys 3.5, there is plenty of help online. I may even write an article or two on getting started with CoDeSys in the future, let me know in the comments what's missing, and I'll update the article

Programming Environment

Every editor is different (Allen Bradley, Mitsubishi, CoDeSys, Siemens for example); however, they all follow the same basic guidelines.

The Ladder environment is made up of Networks (or Rungs) and logic elements that sit on the network from left to right. The logic "flows" from left to right until the rung is complete and then moves downward to the next rung.

Consider this example:

The first network consists of a contact, followed by a negated contact and finally a Coil (or Flag/Output - Not to be confused with a physical output)

The second network consists of a contact and another coil.

In Ladder, the name of the contact, coil or other elements is the assigned variable or "Tag" that is being referenced. So, "Coil" appears twice, once as a Coil and once as a Contact. If a Coil is set to True, a contact with the same name will also be true.

In order for a Coil to be True, everything to the left of the coil must also be in a True state. Think of this as an electrical circuit, where your V-Supply is the vertical rail on the left and on the right is your ground, a coil is a device that requires power and contacts are relays. Some environments actually show a right side rail too.


Remember that the logic flow is across and then down. If a coil on network 5 is set to True, and a contact with the same name is on network 3 and 7, only 7 will be True until the program loops around to the top again.

Flow Example

The above image shows the "logic flow" when the PLC is in run mode. Contacts / Coils that are blue are TRUE, if they are not blue, they are FALSE.

(All variables are set to false in the above image, note that the negated contact is blue)

To set a coil to TRUE (blue), all contacts and conditions to the left of the coil must be TRUE also, allowing the blue path to reach the coil:

In the above logic, "Contact" (the first element on network 1) was set to TRUE. Because "Negated Contact" is a negated contact, FALSE = TRUE. As both conditions are satisfied to the left of "Coil," the Coil is now set to TRUE.

The second network's contact "Coil" is now true. As all conditions to the left of Coil_2 are now TRUE, Coil_2 is set to TRUE.

Function Blocks & Functions

Ladder Logic makes use of Function Blocks and Functions extensively, for the purpose of this introduction we will assume that all Function Blocks & Functions are built using Ladder.

So what's the difference between a Function and a Function Block? It comes down to "Instances." A Function is simply a function, inputs are passed, and an output is given, nothing is held in memory. A Function Block, however, is defined in the program's logic, it's given an instance, and memory is allocated for that block.

Consider you want to do the following mathematical calculation:

X + Y = Z

A Function would provide the means to pass value X and Y and return value Z.

Suppose you wanted to do the following mathematical calculation:

X + Y + Z = Z

A Function Block would allow Z to be defined as an Output, but because the block is given an instance, the value of Z is stored inside the block, so it does NOT need to be passed in again to be used in the calculation. A function block is given an instance by naming it in the same way that a contact or coil is named:

In the above example, a Function Block has been placed on the network. The Function Block has been defined as a TON (Timer On Delay) and has been given the name Timer_1. This has now been allocated memory, Timer_1 is an instance of TON. This allows Timer_1 to use the Functions that TON provide, while keeping track of Timer_1 in its own instance, irrespective of any other TON function blocks.

This is just a brief introduction to function blocks, they are very powerful and need further explanation to understand their full use (look out for an article dedicated to them!)


Ladder Logic is simple, clean and an effective language to program simple and complex control systems with. It is also easy to read, print and follow. It's downfall, in my opinion, is that its very tempting to simply keep adding to the end of a POU or ladder file, creating long and difficult to follow code.

The best use of ladder is for straight logic, basic mapping and small function blocks.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2018 Liam


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