How to Use Case Statement in Structured Text for PLC Programming

In Structured Text, use the CASE statement to execute different code blocks based on the value of a variable. It works like a multi-way branch, checking the variable against multiple OF options and running the matching block. Use ELSE for a default case when no match is found.

Syntax

The CASE statement evaluates a variable and executes the matching OF block. Each OF defines a possible value or range. The ELSE block runs if no values match. The statement ends with END_CASE.

• CASE: starts the case statement with the variable to check.
• OF: defines a value or range to match.
• ELSE: optional default block if no match.
• END_CASE: ends the case statement.

structured-text

CASE variable OF
    value1:
        (* code block 1 *)
    value2..value3:
        (* code block 2 for range *)
    ELSE
        (* default code block *)
END_CASE

Example

This example shows how to use a CASE statement to set a string message based on an integer input value.

structured-text

VAR
    inputValue : INT := 2;
    outputMessage : STRING(20);
END_VAR

CASE inputValue OF
    1:
        outputMessage := 'One';
    2:
        outputMessage := 'Two';
    3..5:
        outputMessage := 'Three to Five';
    ELSE
        outputMessage := 'Other';
END_CASE

Output

outputMessage = 'Two'

Common Pitfalls

Common mistakes when using CASE statements include:

• Forgetting the END_CASE to close the statement.
• Not covering all possible values, which can cause unexpected behavior if ELSE is missing.
• Using overlapping ranges or duplicate values in OF clauses, which is not allowed.
• Assigning values incorrectly inside the case blocks.

structured-text

(* Wrong: Missing END_CASE *)
CASE inputValue OF
    1:
        outputMessage := 'One';
    2:
        outputMessage := 'Two';

(* Correct: *)
CASE inputValue OF
    1:
        outputMessage := 'One';
    2:
        outputMessage := 'Two';
END_CASE

Quick Reference

Keyword	Description
CASE	Starts the case statement with the variable to check
OF	Defines a value or range to match
ELSE	Optional default block if no match is found
END_CASE	Ends the case statement

Key Takeaways

Use CASE to select code blocks based on a variable's value in Structured Text.

Always close the CASE statement with END_CASE to avoid syntax errors.

Include an ELSE block to handle unexpected values safely.

Avoid overlapping or duplicate values in OF clauses.

CASE statements improve readability over multiple IF-THEN-ELSE chains.