Convert Ladder Logic into Structured Text and Functional Block Diagram

12 July 2020.

In this post, we will cover the basic topics on how to convert ladder language logic into structured text and functional block diagram languages. We will use Unity Pro software for study. (Click on the images for a more zoomed view)

Hello programmers. Ladder logic is a very commonly used programming language in industrial automation. In fact, when programmers venture into the world of automation for first time, they are mostly trained in this language as it is simplified and resembles traditional electrical drawing circuits to a very great extent. Over the time, I learnt that though this language is easy to write; it has great drawback of length and online view, which irritates a lot sometimes. Means; if you have written a ladder section comprising around 100 rungs, imagine how much you will have to scroll down and up to view and troubleshoot. Moreover, when many rungs are written in a section, the software takes some amount of time to load the animation when online. This can prove to be a waste if you are in urgent need of finding the bug which comes and moves away instantly, and the software is not able to help you. So, we must adapt to other programming languages for quick troubleshooting and improving the efficiency and view of the program. You can use ladder logic in the program but try to use it only somewhere and not the whole program.

I have come across many freshers who avoid using other languages. They either don’t know how to write their ladder logic in some other language same to same or they are least interested in transforming to some other code and just stick to their basic language. So, I will teach you on how to convert your ladder logic into other language logic and improve your programming standards. Guys, we will cover structured text and functional block diagram languages. These are widely used and overall enhance the view, efficiency and working of the program.

How to convert Ladder Logic to Functional Block Diagram and Structured Text:

Let’s start by referring the ladder logic below. I will consider that the readers are efficient in working in ladder logic; because our focus is to convert and write them in other languages. As you can see, lamp variable turns on when start variable turns on and stop variable is off. This will latch the lamp variable below start variable and will maintain the power of the rung to keep lamp variable on.

Structured Text Programming In PLC:

So, let’s start with structured text language. Structured Text is PLC programming language defined by PLCOpen in IEC 61131-3. The programming language is text-based, compared to the graphics-based ladder diagram or Function Block Diagram. The syntax of this language is made like high-level programming languages like PHP, Python, and C etc., with loops, variables, conditions, and operators. It is important to understand the structure and syntax of the structured text before you proceed ahead. You can then easily understand how the flow of the program works. As you can see in the image below, it runs till the end statement of the program and will continue from start again. It is like ladder language. There are many types of statements used in this language – if-else, case, while, repeat, for etc. We will see if-else statement here.

Read and understand the syntax properly. It is full of colons, semicolons, and other symbols. Each of them has it’s meaning. Semicolon separates a statement and indicates that the end of a statement has been reached. If you are comparing something, then use = symbol; and if you are writing something into a variable, then use: = symbol.

                                              

We will now convert ladder to structured text. Let’s start with the logic now. If you remember the ladder logic that we had seen, lamp variable is latched once start variable turns on. It unlatches only when stop variable turns on and cuts the power of the rung. Now, read and understand how I wrote this in structured text. If start variable turns on, then we are writing value 1 to lamp variable.  Now, when start variable turns off, then too lamp variable will remain on; because the condition to execute the statement is false. This means the lamp variable has been latched now. If stop variable turns on, then we are writing value 0 to lamp variable.  Now, when stop variable turns off, then too lamp variable will remain off; because the condition to execute the statement is false. Then, as there are two if statements; we have terminated them with two end statements. I am thus conveying, that what you wrote in ladder logic; interpret each rung in your mind and make the statements. It’s simple; analyze each element in the ladder rung, understand it’s role and write that same in structured text. One you understand how the final output variable is impacted, then you can easily write the statement.

You can also write this statement as below. In the first image, I have just changed the visualization of the program for better understanding. In the second image, I have replicated the ladder logic in structured text format. Here, I have simply used assignment statement. Means, the final variable lamp will be written as 1 when the whole condition is true and written as 0 when the whole condition is false. So, it all depends on you and how you want to visualize and understand the statement. If you are not familiar with structured text language, you can find many structured text programming pdf files online for understanding.

                      

          

Functional Block Diagram PLC Programming:

Now, let’s start with functional block diagram. It is a simple and graphical way to program any functions together in a PLC program. Most programmers love FBD because it is graphically a very common way to describe a system and they like to put things in boxes. And that is exactly what the concept of function block diagrams is. FBD is nothing but, functions put in function blocks. They all have one or more inputs and outputs. The function of the block is the relation between the state of it’s inputs and outputs.

As you can see in the below image, a function block is shown as a box. In the middle of the box is often a symbol or a text. This symbol represents the actual functionality of the function block. Depending on the function there can be any number of inputs and outputs on the function block. You can connect the output of one function block to the input of another. Thus, a Function Block Diagram is created. FBD looks great in visualization as the whole logic appears in front of your eyes at one go; and helps you in better troubleshooting.

We will now convert ladder to function block diagram. Let’s start with the logic now. As in the image, SR represents SR flip flop logic. When a variable turns on at S1 input (S1 means set), the output Q1 is latched. It will remain latched until the variable at R input (R means reset) turns on and will unlatch the output. So, as per our previous study, I just latched the lamp variable with a start variable at S1 input and unlatched it with a stop variable at R input. 

                                              

You can also write the logic as follows. I have replicated the ladder logic into functional block diagram. I would suggest for first timers; use this logic for understanding. Understand the ladder logic first and convert it as same into functional block diagram. Learn how to link blocks first; because once you can link the blocks and write the logic properly, then it would be easier for you to go for standard and advanced level blocks in the software. Remember guys; always start from scratch.

                

Some important points to remember are:

1. Ladder and functional block programming differ in terms of representation with one using ladder diagram and the other using functional block variables and boxes.

2. Ladder and structured text programming differ in terms of representation with one using ladder diagram and the other using syntax, and statements.

3.  If it is difficult for you as to how to write a logic in programming, first understand the logic and draw it in your own language through flowcharts or free-hand drawings. It helps a lot.

I have covered the general principles which normally are followed to write logic in structured text and functional block diagram. I have also not attempted to cover every type of statements and blocks; you can learn it easily in the software help file or internet. I have just taught you on how to convert and write your ladder logic into some other language. Learn the basics, save time in your coding, make it appear better and improve the efficiency and productivity of your work.

Thank you, guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.