Structured Text (ST)

The structured text consists of a series of instructions which, as determined in high level languages, ("IF..THEN..ELSE") or in loops (WHILE..DO) can be executed.

Example:

IF value < 7 THEN

    WHILE value < 8 DO

    value := value + 1;

    END_WHILE;

END_IF;

Expressions

An expression is a construction which returns a value after its evaluation. Expressions are composed of operators and operands. An operand can be a constant, a variable, a function call, or another expression.

Valuation of expressions

The evaluation of expression takes place by means of processing the operators according to certain binding rules. The operator with the strongest binding is processed first, then the operator with the next strongest binding, etc., until all operators have been processed. Operators with equal binding strength are processed from left to right.

Below you find a table of the ST operators in the order of their binding strength:

Operation	Symbol	Binding strength
Put in parentheses	(expression)	Strongest binding
Function call	Function name (parameter list)
Exponentiation	EXPT
Negate Building of complements	- NOT
Multiply Divide Modulo	* / MOD
Add Subtract	+ -
Compare	<,>,<=,>=
Equal to Not Equal to	= <>
Boolean AND	AND
Boolean XOR	XOR
Boolean OR	OR	Weakest binding

There are the following instructions in ST, arranged in a table together with example:

Instruction	Example
Assignment	A:=B; CV := CV + 1; C:=SIN(X);
Calling a function block and use of the FB version	CMD_TMR(IN := %IX5, PT := 300);A:=CMD_TMR.Q;
RETURN	RETURN;
IF	IF D<0.0 THEN C:=A; ELSIF D=0.0 THEN C:=B; ELSE C:=D; END_IF;
CASE	CASE INT1 OF 1: BOOL1 := TRUE; 2: BOOL2 := TRUE; ELSE BOOL1 := FALSE; BOOL2 := FALSE; END_CASE;
FOR	FOR I:=1 TO 100 BY 2 DO IF ARR[I] = 70 THEN J:=I; EXIT; END_IF; END_FOR;
WHILE	WHILE J<= 100 AND ARR[J] <> 70 DO J:=J+2; END_WHILE;
REPEAT	REPEAT J:=J+2; UNTIL J= 101 OR ARR[J] = 70 END_REPEAT;
EXIT	EXIT;
Empty instruction	;

Instruction in structured text

The name already indicates, the structured text is designed for structure programming, i.e. ST offers predetermined structures for certain often used constructs such as loops for programming. This offers the advantages of low error probability and increased clarity of the program.

For example, let us compare two equally significant program sequences in IL and ST:

A loop for calculating powers of two in IL:

Loop:
LD    Counter
EQ       0
JMPC  
end

LD       
Var1
MUL    
2
ST    Var1

LD       
Counter
SUB    
1
ST    Counter
JMP     
Loop
End:
LD    Var1
ST    ERG

The same loop programmed in ST would produce:

WHILE Counter<>0 DO

    Var1:=Var1*2;

    Counter:=Counter-1;

END_WHILE

Erg:=Var1;

You can see, the loop in ST is not only faster to program, but is also significantly easier to read, especially in view of the convoluted loops in larger constructs.

The different structures in ST have the following significance:

Assignment operator

On the left side of an assignment there is an operand (variable, address) to which is assigned the value of the expression on the right side with the assignment operator :=

Example: Var1 := Var2 * 10;

After completion of this line Var1 has the tenfold value of Var2.

Calling function blocks in ST

A function block is called in ST by writing the name of the instance of the function block and then assigning the values of the parameters in parentheses. In the following example a timer is called with assignments for the parameters IN and PT. Then the result variable Q is assigned to the variable A.

The result variable, as in IL, is addressed with the name of the function block, a following point, and the name of the variable:

CMD_TMR(IN := %IX5, PT := 300);
A:=CMD_TMR.Q

Further Information