Structured Text syntax
Structured text is a textual programming language that uses statements to define what
to execute.
- Structured text is not case sensitive.
- Use tabs and carriage returns (separate lines) to make your structured text easier to read. They have no effect on the execution of the structured text.
Structured text can contain these components.
Term | Definition | Examples |
Assignment | Use an assignment statement to assign values to tags. The :=
operator is the assignment operator. Terminate the assignment with a semi colon ‘;.’ | tag := expression; |
Expression | An expression is part of a complete assignment or construct
statement. An expression evaluates to a number (numerical
expression), or to a true or false state
(BOOL expression) | |
Tag Expression | A named area of the memory where data is stored (BOOL, SINT, INT,
DINT, REAL). | value1 |
Immediate Expression | A literal value | 4 |
Operators Expression | A symbol or mnemonic that specifies an operation within an
expression. | tag1 + tag2 tag1 >= value1 |
Function Expression | When executed, a function yields one value. Use parentheses to
contain the operand of a function. Even though their syntax is similar, functions differ from
instructions in that functions can be used only in expressions.
Instructions cannot be used in expressions. | function(tag1) |
Instruction | An instruction is a standalone statement. An instruction uses parentheses to contain its operands. Depending on the instruction, there can be zero, one, or multiple
operands. Even though their syntax is similar, instructions differ from
functions in that instructions cannot be used in expressions.
Functions can be used only in expressions. | instruction(); instruction(operand); instruction(operand1, operand2,operand3); |
Construct | A conditional statement used to trigger structured text code
(that is, other statements). Terminate the construct with a semi
colon (;). | if...then case for...do while...do repeat...until exit |
Comment | Text that explains or clarifies what a section of structured text
does. Use comments to make it easier to interpret the structured
text. Comments do not affect the execution of the structured text. Comments can appear anywhere in structured text. | //comment (*start of comment . . . end of comment*) /*start of comment . . . end of comment*/ |
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