Structured Text Basics¶

Structured Text (ST) is the primary language supported by IronPLC. This page gives you a conceptual overview of the language so you know what to expect when you start writing code. For hands-on practice, follow the Quick start tutorial.

What Structured Text Looks Like¶

If you have used Pascal, Ada, or even a bit of Python, Structured Text will feel familiar. Here is a small example:

PROGRAM main
   VAR
      Counter : INT := 0;
      Limit : INT := 100;
      Running : BOOL := TRUE;
   END_VAR

   IF Running AND (Counter < Limit) THEN
      Counter := Counter + 1;
   ELSE
      Running := FALSE;
   END_IF;

END_PROGRAM

Some things to notice:

Keywords like PROGRAM, VAR, IF, and THEN are uppercase by convention.
Variable declarations go inside a VAR … END_VAR block at the top.
Assignment uses := (not =).
Statements end with a semicolon.
Blocks are closed with END_PROGRAM, END_IF, and so on rather than curly braces.

Data Types¶

IEC 61131-3 provides a set of elementary data types:

Type	Size	Description
`BOOL`	1 bit	Boolean (TRUE or FALSE)
`INT`	16 bits	Signed integer (-32768 to 32767)
`DINT`	32 bits	Double integer
`REAL`	32 bits	Floating-point number
`STRING`	Variable	Character string
`TIME`	—	Duration (for example, `T#100ms`)

You can also define your own types: enumerations, arrays, structures, and subranges. These are covered in the Data Types.

Control Flow¶

Structured Text supports the control flow statements you would expect:

IF / ELSIF / ELSE:

IF Temperature > 100.0 THEN
   Alarm := TRUE;
ELSIF Temperature > 80.0 THEN
   Warning := TRUE;
ELSE
   Alarm := FALSE;
   Warning := FALSE;
END_IF;

CASE:

CASE State OF
   0: Motor := FALSE;
   1: Motor := TRUE;
   2: Motor := FALSE;
      Alarm := TRUE;
END_CASE;

FOR:

FOR i := 0 TO 9 DO
   Values[i] := 0;
END_FOR;

WHILE:

WHILE Buffer <> 0 DO
   Count := Count + 1;
   Buffer := Buffer / 2;
END_WHILE;

How ST Differs from General-Purpose Languages¶

Structured Text was designed for real-time control, which leads to some differences compared to languages like Python or C:

No dynamic memory allocation. All variables are declared at the top of a block and exist for the lifetime of the program. There is no malloc, new, or garbage collector.
No recursion. Function calls cannot recurse. This keeps execution time predictable.
Cyclic execution. Your program runs repeatedly as part of the scan cycle (see What is IEC 61131-3?). You do not write a main function that runs once — you write logic that runs every scan.
Time literals. Durations are first-class values: T#100ms, T#2s, T#1h30m.

These constraints exist because PLCs must respond within strict time deadlines. Predictability matters more than flexibility.

Next Steps¶

Now that you have a feel for the language, try writing your first program in the Quick start tutorial. When you need precise details about syntax and semantics, consult the Compiler reference.