Structured Text (ST)

ST Structured text - A high-level language that is block structured and syntactically resembles Pascal. is a structured literal programming The act of performing a sequence of instructions or commands. language.

A ST program is a list of statements.
- Each statement describes an action and must end with a semi-colon (;).
The presentation of the text has no meaning for a ST program.
- You can insert blank characters and line breaks where you want in the program text.

Comments

Comment text can be entered anywhere in an ST program.

Comment text:
- Has no meaning for the execution of the program.
- Must begin with (* and end with *).
- Can be entered on several lines (i.e., a comment text can include line breaks).
- Cannot be nested.

Expressions

Each statement describes an action and can include evaluation of complex expressions.

An expression is evaluated:

From the left to the right.
According to the default priority order of operators.
- The default priority can be changed using parentheses Parenthesis ( ).

Arguments of an expression can be:

Declared Variables.

Constant Expressions.

Function Call.

Statements

Basic Statements

These are the available basic statements that can be entered in an ST program:

Assignment := (assignment)
Call a Function Block

Conditional Statements

These are the available conditional statements in ST Language:

Loop Statements

These are the available statements for describing loops in ST Language:

FOR TO BY END_FOR
- Loops with FOR instructions are slow.
  Optimize your code by replacing such iterations with a WHILE statement.

  Iteration of statement execution.
  The BY statement is optional (default value is 1)
FOR iCount := 0 TO 100 BY 2 DO
MyVar := MyVar + 1;
END_FOR;

REPEAT UNTIL END_REPEAT

  Repeat a list of statements.
  Condition is evaluated on loop exit after the statements.
iCount := 0;
REPEAT
MyVar := MyVar + 1;
iCount := iCount + 1;
UNTIL iCount < 100 END_REPEAT;

WHILE DO END_WHILE

  Repeat a list of statements.
  Condition is evaluated on loop entry before the statements.
iCount := 0;
WHILE iCount < 100 DO
iCount := iCount +1;
MyVar := MyVar + 1;
END_WHILE;

Other Statements

These are some other statements in ST Language:

WAIT / WAIT_TIME - Suspend the execution.
ON - Conditional execution of statements: provides a simpler syntax for checking the rising edge The transition of a digital signal from low to high. AKA: positive edge. of a Boolean condition.

ST provides an automatic completion of typed words.
See Auto-completion of Wordsfor more information.