IF statement in ST

Syntax

IF ... THEN
  ...
ELSIF ... THEN
  ...
ELSE
  ...
END_IF;

Meaning

Use the IF statement to specify that a group of statements is to be executed only if the associated Boolean →expression evaluates to value TRUE (or an equivalent). If the condition evaluates to value FALSE (or an equivalent), then either no statement is to be executed, or the statement group following the ELSE keyword (or the ELSIF keyword if its associated Boolean condition evaluates to value TRUE or an equivalent) is to be executed.

Detailed explanation about IF

Part of IF statement	Explanation
`IF condition-A THEN` `statement-A1;` `...` `statement-An;`	If the condition `condition-A = TRUE`, the execution continues with the statement block after `THEN` (of `IF`). The statement block of the `IF`-part may contain as many statements as you like. After processing the last statement, the execution continues after `END_IF`. If the condition `condition-A = FALSE`, the execution continues with `ELSIF`. If there is no `ELSIF`, the execution continues with `ELSE`. If `ELSE` is also missing, the execution continues after `END_IF`.
`ELSIF condition-B THEN` `statement-B1;` `...` `statement-Bn;`	If the condition `condition-B` `= TRUE`, the execution continues with the statement block after `THEN` (of `ELSIF`). The statement block of the `ELSIF`-part may contain as many statements as you like. After processing the last statement, the execution continues after `END_IF`. If the condition `condition-B = FALSE`, the execution continues with the next `ELSIF`. If there is no other `ELSIF`, the execution continues with `ELSE`. If `ELSE` is also missing, the execution continues after `END_IF`. You may skip the `ELSIF`-part or repeat it as often as you like.
`ELSE` `statement-C1;` `...` `statement-Cn;`	If all previous conditions = `FALSE`, the execution continues with the statement block behind `ELSE`. The statement block of the `ELSE`-part may contain as many statements as you like. After processing the last statement, the execution continues after `END_IF`. You may skip the `ELSE`-part.
`END_IF;`	This concludes the `IF`-statement.

Example

FUNCTION_BLOCK ExampleIfDocumentation
 VAR
   up : BOOL;
   count : INT;
 END_VAR
 
 IF up THEN     /* If 'up' = 'TRUE', counter counts up. */
   count := count + 1;
 ELSE
   count := count - 1;
 END_IF;
END_FUNCTION_BLOCK