Command groups

The following command groups are the basic constructs of Structured Text and can be flexibly combined and nested.

Boolean operations

Boolean operations are used for binary linking of variables.

| Symbol | Logical operation | Example | | --- | | --- | |NOT | Binary negation | a := NOT b;| | AND| Logical AND | a := b AND c;| | OR | Logical OR | a := b OR c;| | XOR| Exclusive OR | a := b XOR c;|

The truth table for Boolean operations looks like this:

Input	AND	OR	XOR
0 0	0	0	0
0 1	0	1	1
1 0	0	1	1
1 1	1	1	0

Arithmetic operations

The Structured Text contains basic arithmetic operations for use. The priorities are to be observed during execution.

Symbol	Arithmetic operation	Example
:=	Allocation	a := b;
+	Addition	a := b + c;
-	Subtraction	a := b - c;
*	Multiplication	a := b * c;
/	Division	a := b / c;
MOD	Modulo, integer remainder of division	a := b MOD c;

The data type of variables and values is always crucial for their calculation.

| Expression / Syntax | Var1 | Var2 | Var3 | Result | | --- | --- | --- | -- | | Var 1 := 10 / 4; | INT | INT | 2 | | Var1 := 10 / 4; | REAL | INT | 2.0 | | Var1 := 10.0 / 4; | REAL | REAL | INT | 2.5 |

Comparison operators

Symbol	Comparison expression	Example
=	Anon	IF a = b THEN
<>	Unequal	IF a <> b THEN
>	Larger than	IF a > b THEN
>=	Greater than or equal to	IF a >= b THEN
<	Smaller than	IF a < b THEN
<=	Less than or equal to	IF a <= b THEN

Decisions

IF a > b THEN // 1. Comparison
c := 1; // statement if 1. Comparison TRUE
ELSIF a > d THEN // 2. Comparison
e := 1; // statement if 2. Comparison TRUE
ELSE // Alternative branch, no comparison TRUE
f := 1; // Statement of the alternative branch
END_IF // End of the decision

IF - ELSE statement

IF - ELSIF instruction

CASE instruction

The CASE statement is used to group multiple conditional statements with the same conditional variable.

CASE newCase OF // start of Case         
    1,5: // for 1 and 5
        StateDescription := "Stopped";      
    2: // for 2
        Statedescription := "Running";
    3, 4, 6 ... 8: // for 3, 4, 6, 7, 8
        Statedescription := "Failed";
ELSE // Alternative branch
    (* .. *)
END_CASE // End of Case

In a program cycle, only one step of the CASE instruction is processed at a time. The step variable must be an integer data type.

Grinding

Loops are processed repeatedly within a cycle. The code is executed until a defined termination condition is met.

To avoid infinite loops, a way should always be provided to end the loop after a certain number of repetitions.

Header controlled loops (FOR, WHILE) check the termination condition before the run, footer controlled loops (REPEAT) at the end.

FOR statement

The FOR instruction is used to execute a certain number of repetitions of a program part.

Sum := 0;
FOR Index := 0 TO 3 DO
    Sum := Sum + Values[ Index ];
END_FOR;

WHILE statement

The WHILE statement does not have a loop counter. This is called until a condition or expression is FALSE.

WHILE Index < 10 DO
    Sum := Sum + Values[ Index ];
    Index := Index + 1;
END_WHILE;

REPEAT instruction

The termination condition is checked in the REPEAT loop only after execution.

Index := 0;
Sum := 0;
REPEAT
    Sum := Sum + Values[ Index ];
    Index := Index + 1;
UNTIL Index >= 10 END_REPEAT;

EXIT statement

Can be used with all loop types and results in immediate termination.

REPEAT
    IF Exit = TRUE THEN 
    EXIT; 
    END_IF
    UNTIL Index >5
END_REPEAT