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IF...THEN...ELSE Statements

The IF statement is the primary way to implement conditional logic in Structured Text. It allows the program to make decisions based on conditions that evaluate to 1 (taken) or 0 (not taken).

Basic IF Statements​

A simple IF statement executes one or more statements only when the condition evaluates to 1.

Syntax:​

IF <condition> THEN
    <statements>;
END_IF;

Example:​

IF Start_Button THEN
    Motor_Start := 1;
END_IF;

IF...THEN...ELSE​

You can extend an IF statement with an ELSE block to handle the case where the condition evaluates to 0.

Syntax:​

IF <condition> THEN
    <then_statements>;
ELSE
    <else_statements>;
END_IF;

Example:​

IF Start_Button THEN
    Motor_Start := 1;
ELSE
    Motor_Start := 0;
END_IF;

This structure ensures the motor starts only when the button is pressed, and stops otherwise.

IF...ELSIF...ELSE​

For multiple conditions, use ELSIF (else-if). This is helpful for multi-way logic.

Syntax:​

IF Mode = 1 THEN
    Operation := 1;
ELSIF Mode = 2 THEN
    Operation := 2;
ELSE
    Operation := 0;
END_IF;

Each condition is evaluated in order, and only the first matching block is executed.

Notes​

  • Every IF block must be closed with END_IF;

  • Each line inside the block must end with a semicolon

  • You can nest IF statements, but keep them readable

More Examples​

// Turn on fan only if temperature exceeds limit
IF Temperature > Temp_Limit THEN
    Fan := 1;
ELSE
    Fan := 0;
END_IF;

// Set alarm mode based on pressure
IF Pressure > 100 THEN
    Alarm_Mode := 2;
ELSIF Pressure < 30 THEN
    Alarm_Mode := 1;
ELSE
    Alarm_Mode := 0;
END_IF;