Structured Text (ST)

ST is a structured literal programming 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.

Arguments of an expression can be:

Statements

Basic Statements

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

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 of a Boolean condition.

See Also

Structured Text (ST) / Instruction List (IL) Editor

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