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ST Code Objects

This topic describes the following code objects that can be used when programing ST:

Expressions

Expressions are parts of statements. An expression returns exactly one value which is then used for the execution of the statement where the expression is used.
Each expression consists of operands and operators (see below) or functions calls.

Operands and operators

Operands and operators are both parts of ST expressions.
Operands can be literals, variables or names of functions. Operands are logically combined using operators , i.e., operators are applied to operands.
In an expression, the operator with the highest priority is executed first, followed by the operator with the next lower priority. The following ST operators are defined.

Operator	Example	Value of example	Description	Priority
( )	(2+3)* (4+5)	45	Parenthesization	Highest
Function name (parameter list)	MAX(4,10)	10	Function call	1
- NOT	-10 NOT TRUE	-10 false	Negation Complement	1
**	3.0**4	81.0	Exponentiation	1
* / MOD	10*3 6/2 17 MOD 10	30 3 7	Multiplication Division Modulo	1
+ -	2+3 4-2	5 2	Addition Subtraction	1
<, >, <=, >=	4 > 12	false	Comparison	1
= <>	T#26h = T#1d2h 8 <>16	true true	Equality Inequality	1
&, AND	TRUE & FALSE	false	Boolean AND	1
XOR	TRUE XOR FALSE	true	Boolean exclusive OR	1
OR	TRUE OR FALSE	true	Boolean OR	Lowest

Statements

Note Depending on the used target, the nesting level of statements may be limited. Furthermore, there may exist some other controller-specific rules.

Keyword	Example	Description
:=	variableName := expression;	Assignment statement The simplest ST statement type. It copies the value of the expression on the right to the variable on the left. Both the variable on the left and the value of the expression on the right must have the same data type. Otherwise, a type conversion must be used before.
RETURN	RETURN;	Return statement The return statement exits the called function, function block or program and returns to the calling POU.
IF	IF a < b THEN c:=1; ELSIF a=b THEN c:=2; ELSE c:=3; END_IF;	Selection statement A group of statements is executed only if the associated Boolean expression 'a<b' is TRUE. If the condition is FALSE, either no statement is executed or the group of statements following ELSE is executed.
CASE	CASE f OF 1: a:=3; 2..5: a:=4; 6: a:=2; b:=1; ELSE a:=0; END_CASE;	Selection statement A group of statements is executed according to the value of the expression following the keyword CASE. The variable or expression 'f' must be of the data type ANY_INT.
FOR	FOR a:=1 TO 10 BY 3 DO f[a] :=b; END_FOR;	Iteration statement A group of statements is executed repeatedly increasing the variable 'a' by 3, beginning at '1' and finishing at '10'. The starting point is indicated by the assigned value of the control variable 'a'. The final value is indicated following 'TO' and the increments are indicated following 'BY'. All values must be of the same ANY_INT data type. Note If 'BY' is not specified, the default value '1' is used. All values must then be INT data types.
WHILE	WHILE b > 1 DO b:= b/2; END_WHILE;	Iteration statement A group of statements is executed repeatedly until the associated Boolean expression 'b>1' is FALSE. The statement's exit condition is evaluated at the beginning of the loop. If the condition is FALSE, the loop is not executed.
REPEAT	REPEAT a := a*b; UNTIL a <  10000 END_REPEAT;	Iteration statement A group of statements is executed repeatedly until the associated Boolean expression 'a<10000' is TRUE. The statement's exit condition is evaluated at the end of the loop. If the condition is FALSE, the loop is executed at least once.
EXIT	FOR a:=1 TO 2 DO IF flag THEN EXIT; END_IF SUM:= SUM + a END_FOR	Exit statement The exit statement can be used to abort the execution of an iteration statement.