How to Use If Else in Structured Text for PLC Programming

In Structured Text, use IF to check a condition and ELSE to run code when the condition is false. The syntax is IF condition THEN ... ELSE ... END_IF; to control program flow based on conditions.

Syntax

The IF statement checks a condition. If true, it runs the code after THEN. If false, it runs the code after ELSE. The statement ends with END_IF;.

• IF: starts the condition check
• condition: a true/false expression
• THEN: code to run if condition is true
• ELSE: code to run if condition is false (optional)
• END_IF;: ends the if statement

structured-text

IF condition THEN
    (* code if true *)
ELSE
    (* code if false *)
END_IF;

Example

This example checks if a temperature is above 30 degrees. If yes, it sets a fan to ON; otherwise, it sets the fan to OFF.

structured-text

VAR
    temperature : INT := 35;
    fan : BOOL := FALSE;
END_VAR

IF temperature > 30 THEN
    fan := TRUE;
ELSE
    fan := FALSE;
END_IF;

Output

fan = TRUE

Common Pitfalls

Common mistakes include forgetting END_IF;, missing THEN, or using assignment = instead of comparison =:= in some PLCs. Also, not using parentheses for complex conditions can cause errors.

structured-text

(* Wrong: Missing THEN and END_IF *)
IF temperature > 30
    fan := TRUE
ELSE
    fan := FALSE

(* Correct: *)
IF temperature > 30 THEN
    fan := TRUE;
ELSE
    fan := FALSE;
END_IF;

Quick Reference

Keyword	Purpose
IF	Start condition check
THEN	Code to run if condition is true
ELSE	Code to run if condition is false (optional)
END_IF	End the if statement

Key Takeaways

Use IF ... THEN ... ELSE ... END_IF; to control flow based on conditions in Structured Text.

Always include END_IF; to close the if statement.

Use comparison operators correctly and include THEN after the condition.

ELSE is optional but useful for handling false conditions.

Parentheses help clarify complex conditions and avoid errors.