Structured Text Programming: A Step by Step Guide (With Examples)

Example:

TEMPERATURE := 93.9; TEMPERATURE >= 100.0

Result:

Logical Operators

• & or AND
• OR
• XOR
• NOT

Example:

LIMIT_SWITCH1 := TRUE; LIMIT_SWITCH2 := FALSE; LIMIT_SWITCH1 OR LIMIT_SWITCH2

Result:

Bitwise Operators

• & or AND
• OR
• XOR
• NOT

Example:

15 AND 8

Result:

Since this operation is bitwise the calculation will be per bit. So to understand what’s going on here, you have to convert the numbers to binary values:

15 = 1111
8 = 1000

Now each bit in the number 1111 (15) can be used in a logical operation with the other number 1000 (8):

1111 AND 1000

Operators and Statements

So, in the previous section, you learned that expressions evaluate. Meaning that all expressions will yield the result and the compiler will replace the expression with the result.

But what if you want the PLC (compiler) not to evaluate something, but to DO something?

Statements are the answer.

As I mentioned previously in this article, statements are you telling the PLC what to do. It’s the instruction you give the PLC to take action.

If you make an expression that yields a result, that won’t do much. Expressions are all the calculations and if you don’t use the results of those expressions in some actions (statements), it will be like buying groceries but not cooking.

Let’s take a look at the actions or statements that you can make in Structured Text.

Assignment Statement and Operator

There are several statements available in Structured Text. All of them represent an action or a condition.

Beginning with actions, the most fundamental statement in Structured Text is the assignment statement. Statements are also described in the IEC standard developed by PLCOpen, and the first one they list is the assignment statement.

Here’s how an assignment statement looks like:

A := B;

What does this statement tell the compiler to do?

To take the value of the variable B and put it in the variable A.

The PLC is assigning a value to a variable. Here’s an even simpler example:

A := 10;

This statement will take the value 10 and put it into the variable A. Or said in another way – the variable A will be assigned the value 10.

Since the value of A is now 10, we can make another statement, but this time with an expression:

B := A + 2;

When this line of code is compiled, the expression A + 2 will be evaluated to 12. The compiler will replace the expression with the result 12. The statement will now look like this to the compiler:

B := 12;

What will happen now, is that the compiler will assign the value 12 to the variable B.

How an assignment statement with expression will be evaluated by the compiler.

The last thing is that the := symbol is called the assignment operator. Yes, it is an operator just like the operators used in expressions. Often those two types of operators are mistaken for each other and used wrong.

A common mistake is to use the equality operator (=) instead of the assignment operator (:=). But even though they look like each other there’s a huge difference. Take these two examples:

A = B

A := B;

The first line is an expression. Since this is an expression, the operator will be used to evaluate the line. The equality operator evaluates in the following way:

If the right side and the left side is equal it evaluates to TRUE or 1. If not, it will evaluate to FALSE or 0.

With some other operators, the equality operator is a relational operator. All the relational operators will evaluate to either TRUE or FALSE.

On the second line, you’ll see a statement. This time the operator will be used for action instead of an evaluation. The assignment is the action, and here the value of A will be given the value of B.

At last, you can always identify a statement by the semicolon. Once again, the semicolon is how the compiler knows when the end of a statement is reached.

You can have all sorts of expressions in your assignment statements, from simple values like numbers to variables and functions. Because all expressions will be evaluated first, and then, the result of that evaluation will be used in the assignment statement.

Conditional Statements

Well, the assignment statement was pretty simple: Take the value of the right side and store it in what’s on the left side.

But let’s zoom out a bit and think about PLC programs. A PLC program is a piece of logic (I call it PLC logic) and therefore has to make some decisions. That’s why we use a PLC or any other controller. To decide and act on the current state.

Simplified: The PLC will look at the states of all the inputs and use your PLC program to decide what outputs to set.

So in your PLC program, you need a way to make decisions. This brings us to conditional statements.

Conditional statements are used for exactly that: To make decisions.

There are two ways of doing conditional statements in Structured Text: IF statements and CASE statements.

IF Statements

I think Bill Gates is better at explaining the IF statement than I am. At least he can explain it in just over 1 minute in this great video from code.org. You can skip the video if you are familiar with IF statements, although I would recommend that you watch it.

IF statements are decisions with conditions.

But even though IF-statements are quite simple to understand, you still have to know how to give the PLC the conditional statements. This brings us back to the syntax.

There’s a special syntax for IF statements. This means, that you have to write it in a certain way for the compiler to understand it. Because just like semicolons are used to end statements, there are special keywords to make an IF statement.

Here’s how the syntax for IF statements look like in STL:

IF [boolean expression] THEN ; ELSIF [boolean expression] THEN ; ELSE ; END_IF ;

Notice that the syntax for IF statements look very similar to plain English. The first line contains two keywords: IF and THEN. Between those two keywords is the condition, which is an expression. But not just any expression. A boolean expression.

Boolean and Numeric Expressions

You can divide expressions into two groups depending on what they yield.

Boolean expressions evaluates to a BOOL type value, TRUE or FALSE.

Here’s an example of a boolean expression:

1 = 1

This expression will evaluate to or yield TRUE. A boolean expression could also look like this:

1 > 2

But this time the boolean expression will evaluate to FALSE, since 1 is not larger than 2.

Numeric expressions evaluates to an integer or a floating point number.

A numeric expression could look as simple as this one:

13.2 + 19.8

This expression will evaluate to the floating-point number 33.0, and therefore is a numeric expression.

Boolean expressions are used in IF statements as conditions.

IF the boolean expression evaluates to TRUE, THEN the following statements will be executed.

The PLC will only execute the statements after the keyword THEN, if the expression evaluates to TRUE. This is illustrated by the following example:

A := 0; IF A = 0 THEN B := 0; END_IF ;

Line number 3 will only be executed if A is equal to 0. In this case it will. A 0 is assigned to the variable A in a statement right before the IF statement.

See what I just did here?

In the example above a decision was made depending on the value of a variable. Now, even though this was a fairly simple decision, we can already translate that into real PLC programming.

Let’s say you want to make a program that sets a PLC output depending on the state of input. With a simple IF statement you can do that in Structured Text:

IF INPUT1=TRUE THEN OUTPUT1 := TRUE; END_IF;

Although this example is just a piece of a bigger program (the variable INPUT1 represents an input and OUTPUT1 an output) it illustrates how the decision for a PLC output can be made. The OUTPUT1 variable will only be set to TRUE IF the INPUT1 variable is TRUE.

Since, both the INPUT1 and OUTPUT1 variables are of the type BOOL, the first line in the statement could also look like this:

IF INPUT1 THEN

Just writing the expression as “INPUT1” will still evaluate as TRUE, when the variable is TRUE.

What ELSE IF not?

For now, you’ve seen a simple IF statement, where statements are only executed if an expression is TRUE. If that expression evaluates to FALSE the statements will simply not be executed.

But what if your PLC program requires multiple conditions?

Of course, you could write this as multiple individual IF statements. But Structured Text has more options for the IF statements.

Just like most other programming languages you can use the ELSIF and ELSE keywords for multiple conditions in the same IF statement.

Both ELSIF and ELSE are optional in IF statements, but this is how the syntax looks like:

IF [boolean expression] THEN ; ELSIF [boolean expression] THEN ; ELSE ; END_IF ;

If the boolean expression on line 1 is FALSE, the statements below will simply not be executed. Instead, the compiler will check the boolean expression after the ELSIF keyword.

Here it works just like with the IF keyword: If the boolean expression after the keyword is true, the following statements will be executed.

At last, is the ELSE keyword. It works as a default option for your IF statement. If all the IF and ELSIF boolean expressions are evaluated to FALSE, the statements after the ELSE keyword will be executed.

How the PLC will execute IF statements in Structured Text.

Combining Operators for Advanced Conditions

Besides making multiple conditions you can also expand your conditions to include multiple variables. You can combine multiple expressions, typically done with a logical operator, to get a larger expression.

What if you want not just 1 but 2 inputs to be TRUE before an output is set. The expression would look like this:

IF (INPUT1) AND (INPUT2) THEN OUTPUT1 := TRUE; END_IF;

Now the expression will evaluate to TRUE, only if INPUT1 and INPUT2 is TRUE.

CASE Statements

The second way of making decisions in Structured Text is with CASE statements.

Essentially, CASE statements and IF statements are the same. But CASE statements use numeric expressions instead of boolean expressions. CASE statements also have a slightly different syntax, which makes it more suitable for certain purposes.

This is how the syntax for CASE statements looks like in Structured Text:

CASE [numeric expression] OF result1: ; resultN: ; ELSE ; END_CASE;

In CASE statements there is only 1 expression. The result of that expression is then used to decide which statements are executed.

As a default option, CASE statements also have an ELSE keyword. The statements after that keyword are executed only if none of the results (or cases) matches the result of the numeric expression.

Here’s a very simple example:

PROGRAM_STEP := 3; CASE PROGRAM_STEP OF 1: PROGRAM_STEP := PROGRAM_STEP+1; 2: PROGRAM_STEP := PROGRAM_STEP+2; 3: PROGRAM_STEP := PROGRAM_STEP+3; ELSE PROGRAM_STEP := PROGRAM_STEP+10; END_CASE;

Although this is a very simple example (the variable has a fixed value) the example shows you how to make a decision depending on the result of a numeric expression. In this example, the numeric expression is simply just the value of the variable, 3. If could be any expression that evaluates to an integer or a floating-point value.

Iteration with Repeating Loops

Probably one of the most powerful features in Structured Text is the ability to make loops that repeat lines of code.

Once again, Code.org has made one of the best introductions to repeating loops. This time, Facebook founder, Mark Zuckerberg uses a little more than a minute to explain repeating loops.

In relation to PLC, programming loops can be used for many different purposes. You might have a function or a set of statements that you want to execute a certain amount of times or until something stops the loop.

1. FOR
2. WHILE
3. REPEAT

Common for all the types of loops is that they have a condition for either repeating or stopping the loop. The condition in FOR and WHILE loops decides whether the loop should repeat or not. But for the REPEAT loop the condition is an UNTIL condition, and it will decide whether the loop should stop or not.

FOR Loops

The first loop is the FOR loop and is used to repeat a specific number of times. FOR loops has some other keywords. TO, BY, DO and END_FOR.

This is the syntax of FOR loops in Structured Text:

FOR count := initial_value TO final_value BY increment DO ; END_FOR;

At first sight the first line looks a bit complicated, but it isn’t if you divide it in chunks:

FOR
Keyword that starts the FOR loop statement.

count := initial_value
This assignment operation is where you set the initial value you want to count from. The count is the variable name and initial_value is the value you want to start counting from.

TO
The keyword before the value to count up to.

final_value
This is the value you want to count to. Place 100 here and your loop will count up to 100.

BY
Keywords to use custom incremental value.

increment
The value of which you want to increase the count for every time the loop runs. If you set the increment to 10 and the count to 100, the loop will run 10 times.

DO
;
END_FOR;
This last part between the keyword DO and END_FOR is the statements you want to execute each time your loop runs. These statements will be executed as many times as the loops repeat.

Since FOR loops can only have a preset amount of time they will repeat, that is what they are used for. In PLC programming this could be something as simple as an item that has to be painted/dried four times. A FOR loop that counts to four will work just fine here.

At last, you can use an IF statement with the keyword EXIT to stop the loop before the count. You can add a boolean condition that if TRUE stops the loop.

IF [boolean expression] THEN EXIT; END_IF;

WHILE Loops

The while loop is a little different from the FOR loop, because it is used to repeat the loop as long as some conditions are TRUE. A WHILE loop will repeat as long as a boolean expression evaluates to TRUE.

Here’s the syntax of WHILE loops:

WHILE [boolean expression] DO ; END_WHILE;

Between the WHILE and DO keywords are the boolean expression. If that boolean expression evaluates to TRUE, all the statements until the END_WHILE keyword will be executed.

When END_WHILE is reached, the boolean expression will be evaluated again. This will happen over and over again until the expression doesn’t evaluate to TRUE. But to make the loop stop at one point, you have to change a value in the boolean expression. Only in that way can the boolean expression go from TRUE to FALSE.

Here’s an example of a WHILE loop in Structured Text:

counter := 0; WHILE counter < 10 DOcounter := counter + 1; machine_status := counter * 10; END_WHILE;

If you look at the third line you will see how the loop will eventually stop repeating. The boolean expression uses the counter variable and checks if its value is less than or equal to 10. But since the value of the counter is set to 0 right before the WHILE loop, the boolean expression will be TRUE unless the counter is changed.

That is what’s happening in line 3. This is the first statement in the WHILE loop, and with the other statements, are executed each time the loop repeats. In the third line the value of the counter variable is increased by 1. You can say that the incremental value is 1.

In the example above, the loop will repeat 10 times. When the value of count reaches 10, the boolean expression will be evaluated to FALSE (because 10 is not less than 10) and the loop will stop.

You can also use the EXIT keyword in the WHILE loop to stop repeating the loop before the boolean expression is FALSE. The syntax is an IF statement with the EXIT keyword in. Place it anywhere between DO and END_WHILE keywords.

IF [boolean expression] THEN EXIT; END_IF;

REPEAT Loops

The last type of repeating loop in Structured Text is the REPEAT loop. It works the opposite way of the WHILE loop. This loop will stop repeating when a boolean expression is TRUE.

In ST, the syntax for REPEAT loops looks like this:

REPEAT ; UNTIL [boolean expression] END_REPEAT;

Notice here that since the boolean expression in this type of loop is after the statements, the statements will always be executed at least one time. This is useful if you want an action to happen one time and then, with a condition, decide if that action should happen again.

Just as with the WHILE loops you have to change a value in the boolean expression along the way, to make the loop stop repeating. This can be done be incrementing the value of a variable (to count), or it can be done with a conditional statement like an IF statement inside the loop.

Structured Text Programming Software

Now, even if you have read this article in detail, you’ve only started learning Structured Text. What you should do now is get your hands in the dirt and start using Structured Text.

You should write some PLC programs. Because that is the way to really learn Structured Text and master the programming language.

Beckhoff TwinCat 3

One of the best pieces of PLC programming software, when you want to learn Structured Text, is Beckhoff TwinCat 3. The programming software from Beckhoff is fully compatible with all the IEC 61131-3 PLC programming languages including Ladder Diagram (LD) and Structured Text (ST).

For learners, the biggest advantage of TwinCat 3 is that it has a simulator included. You don’t need to buy a PLC, you just use the soft PLC.

On YouTube there is a free series of videos from SquishyBrained. You should follow him! He even made a video series about his DIY 3D Printer. It’s a great video tutorial to get you started with Structured Text PLC programming in TwinCat 3.

Codesys

You may have heard of Codesys before. It is an open-source software environment for IEC 61131-3 PLC programming. Open source just means that it is free for download, which makes it perfect for students.

Although there are not many great resources on how to use Codesys for beginners, Brian Hobby has made some amazing tutorial videos.

The first video shows you how to create a new project in Codesys. A little Ladder Logic is also included in the video.

The second video helps you program in Structured Text with Codesys.

Conclusion

Learning a new programming language can be quite a challenge. But for beginners there are some very basic things you should always keep in mind:

• Learning takes time
  You just started out. Give yourself some time to learn the language (syntax, functions, …)
• Practice as much as possible
  Try to make as many PLC programs and solutions in Structured Text as possible.
• Learn from your failures
  Every time you make a mistake, don’t get sad. Learn from it, and become a better programmer.
• Keep learning
  Never stop reading, watching tutorials, and other learning materials.
• Talk to other PLC programmers
  Last but not least is discussing in forums and asking questions. Get involved and learn from other PLC programmers.

I think, that the last part is the most important part. Learning from others with experience can be the most effective way to learn, not just about the programming language, but how to use it.

Join the discussion below: Ask your first question about Structured Text and connect with other PLC programmers.

What did you learn from this tutorial?

Good overview, will probably point some of the Engineering Institute of Technology students this way as it’s great intro topic.

Thanks a lot Brian! I am glad that your students can use this resource.

Finally I have found one of the best and nicely written tutorial on structured text. Great Work .

Great tutorial for beginners…. Thanks

Hi
First thank you for your great tutorial
I want to ask you if its possible email me this tutorial( stl language) in pdf
Thank you so much

Thanks you very much for your comment! I’m glad that you like my tutorial. I have added a link to the pdf version in the top of the article.

This is a really good introduction to Structured Text Language. Just a note to remember that every PLC vendor is allowed to have a slightly different “flavor” of syntax – the IEC-61131-3 spec only nails down data types, and then says what the capabilities of the language must be, not the syntax.

However, I take exception to this: “The truth is, that even though ladder diagram (LD) is an easy programming language for beginners, it can be very hard to read and understand.”

The truth is that every programming language is difficult for beginners to understand because code is a lot harder to read than it is to write. This is no different than English – my 5 year old can write sentences, but still needs a lot of help reading most words, and even then will find it hard to get the meaning of a paragraph after getting bogged down in comprehending the individual words.

I have written programs in BASIC, C/C++, Assembly, VB6, VB.Net, C#, Ladder and Structured Text. Of these I’m proficient in VB6, C#, and Ladder because I use them almost daily. I assure you that I can comprehend someone else’s Ladder Logic faster than I can read a Structured Text program, and that’s simply due to experience and practice.

Even so, there are two reasons why Ladder Logic dominates in control system programming (at least in North America). First, as you’ve alluded to, electricians and technicians already “get it” so they immediately get a bump up the experience ladder, and that’s a great thing! I don’t know about you, but at 2 am, I’d rather the night shift electrician be able to deal with the problem right then and there without waking up someone in engineering. 🙂 Secondly, traditional imperative programming, like Structured Text (i.e. do this, then do that, then that) is a less useful mental model to have about a machine. Many things in a machine are happening in parallel. Since Ladder Logic is mimicking electrical relays, which also intuitively operate in parallel, this is a more apt mental model of what’s going on.

If I’m programming an Arduino in C++, my knowledge of Ladder Logic helps me to write better C++ automation code. I’ve said before that Structured Text is an imperative language and Ladder Logic is a functional programming language, and there’s a reason functional programming is all the rage these days. Functional programming is easier to reason about. At its core, a PLC operates by computing the outputs and next scan’s internal state based on the inputs and last scan’s internal state. If you follow a few basic rules like making sure that each output and internal coil is only assigned once per scan, then you’re doing functional programming.

Basically, when someone says, “It can be almost impossible to find head and tail in a larger PLC program written in ladder logic,” my years of experience has taught me that person just doesn’t have experience with ladder logic programming, and were frustrated the first time they saw it. I can’t make heads nor tails of Japanese texts either, but if I claimed this meant English is superior to Japanese, clearly that would be a fallacy, and my error would be all the more apparent to a Japanese person.

That said, you have a very impressive site here full of useful information. Thanks so much for taking the time to add to the resources available online for beginners. I’m glad the new generation won’t have to spend so much time coming up to speed as we did!

I would like to also point out that it is a fallacy to say Ladder vs Structured Text. It depends on the task. If I need to handle some arrays with some kind of iterative loop then I’ll use Structured Text. If I need to look at 10 switches and do some kind of simple output logic then relay logic. One thing about relay logic is that you can look at it and see all on/off states which makes trouble-shooting very fast.

Thank you for the tutorial / overview! I’ve been using structured text for a while now, and actually prefer it to ladder logic (which I have used much longer). I agree that, at first glance, ladder might appear to be a more understandable language. Where it is lacking is in program control and, (no pun intended), structure, which is critical to having code that is easily maintained. While clear and structured ladder can be written, it is more often than not poorly implemented, or often ends up so after feature creep. Simply put, I believe it takes more discipline to write good ladder than it does to write good structured text.
I also find ST to be much more readable. When I read ladder, what I am thinking is often close to the language that is native in structured text… “if THIS, then THAT”. Having it in word form removes a step and allows me to focus more clearly on solving the problem, rather than on the language and tools. Also, being text based, much more code is visible at a given time on the screen, so no constant scrolling.

Minor error, I believe. Your BITWISE operator statement initial example seems to say that ANDing 15 and 8 equals 15. I’m pretty sure that’s supposed to equal 8. Your explanation even seems to indicate it should be 8, but the first part says the result is 15.

Otherwise, this is a great tutorial. I’ve used ST off and on for years at work, but always wished I had a better introduction. This is perfect.

Pretty nice for beginners. I found this tutorial to be very helpful.

I found a litte mistake under the Bitwise Operator Heading.

The table should be like this:

Bit Number 1111 (15) 1000(8) Result
0 1 0 0
1 1 0 0
2 1 0 0
3 1 1 1

Hi.. The Tutorial is good.. Thank You..
Also can i u send me a program in structured text in the right syntax.. I just need to compare with other programming techniques and it will help my students a lot…

Load Binary values A and B..
Logical Operation to be performed – A “AND” B
Result should be stored in C.

My mail Id is [email protected]
Thanks in Advance..
Revin.

Hi, you mention that it’s possible to have function blocks containing functions written in Structured Text. Do you have an example of this?

is there a program out there that can directly convert LD to ST?