How to Declare Variables in Structured Text for PLC Programming

In Structured Text, declare variables using the VAR block followed by the variable name, a colon, and the data type, ending with a semicolon. For example, VAR myVar : INT; declares an integer variable named myVar. Variables must be declared before use inside a program or function block.

Syntax

Variables in Structured Text are declared inside a VAR block. Each variable declaration includes the variable name, a colon, the data type, and ends with a semicolon. The block is closed with END_VAR.

• VAR: starts the variable declaration section.
• variable_name: the name you choose for your variable.
• :: separates the name and type.
• data_type: the type of data the variable holds (e.g., INT, BOOL, REAL).
• ;: ends the declaration line.
• END_VAR: ends the variable declaration section.

structured-text

VAR
    myInteger : INT;
    myBoolean : BOOL;
    myReal : REAL;
END_VAR

Example

This example shows how to declare variables and assign values in Structured Text. It declares an integer and a boolean, then sets the boolean based on the integer's value.

structured-text

PROGRAM ExampleProgram
VAR
    counter : INT;
    isActive : BOOL;
END_VAR

counter := 10;
isActive := counter > 5;

Output

counter = 10
isActive = TRUE

Common Pitfalls

Common mistakes when declaring variables in Structured Text include:

• Forgetting the VAR and END_VAR keywords around declarations.
• Missing semicolons at the end of each declaration line.
• Using invalid variable names (e.g., starting with a number or using spaces).
• Not declaring variables before using them in code.

Always check your syntax carefully to avoid these errors.

structured-text

(* Wrong: Missing END_VAR and semicolon *)
VAR
    temp INT

(* Correct: Proper declaration *)
VAR
    temp : INT;
END_VAR

Quick Reference

Element	Description	Example
VAR	Starts variable declarations	VAR
variable_name	Name of the variable	counter
:	Separates name and type	:
data_type	Type of variable	INT, BOOL, REAL
;	Ends declaration line	;
END_VAR	Ends variable declarations	END_VAR

Key Takeaways

Always declare variables inside a VAR...END_VAR block before using them.

Each variable declaration must include a name, colon, data type, and semicolon.

Use valid variable names starting with letters and no spaces.

Common errors include missing semicolons and forgetting END_VAR.

Structured Text supports common data types like INT, BOOL, and REAL.