IF instruction

With the IF instruction you can check a condition and, depending upon this condition, execute instructions.
Syntax:

IF <Boolean_printout1> THEN

<IF_instructions>

{ELSIF <Boolean_printout2> THEN

<ELSIF_instructions1>

.

.

ELSIF <Boolean_printout n> THEN

<ELSIF_instructions n-1>

ELSE

<ELSE_instructions>}

END_IF;

The part in braces {} is optional.

If the <Boolean_printout1> returns TRUE, then only the <IF_Instructions> are executed and none of the other instructions. Otherwise the Boolean expressions, beginning with <Boolean_printout2>, are evaluated one after the other until one of the expressions returns TRUE. Then only those instructions after this Boolean expression and be-fore the next ELSE or ELSIF are evaluated. If none of the Boolean expressions produce TRUE, then only the <ELSE_instructions> are evaluated.

Sample:

IF temp<17

THEN heating_on := TRUE;

ELSE heating_on := FALSE;

END_IF;

Here the heating is turned on when the temperature sinks below 17 degrees. Otherwise it remains off.