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Operator: OR_ELSE dl-permalink__symbolPermalink

The operator is an extension of the IEC 61131-3 standard.

The ‎OR_ELSE‎ operator is permitted only for programming in structured text: ‎OR‎ operation of ‎BOOL‎ and ‎BIT‎ operands with short-circuit evaluation. This means:

When at least one of the operands yields ‎TRUE‎, the result of the operation also yields ‎TRUE‎; otherwise ‎ FALSE‎.

In contrast to using the ‎OR‎ IEC operator, for ‎OR_ELSE‎ the expressions on all other operators are not evaluated as soon as one of the operands is evaluated as ‎TRUE‎.

Example 

VAR
        bEver: BOOL;
        bX: BOOL;
        dw: DWORD := 16#000000FF;
END_VAR

bEver := FALSE;
bX := dw.8 OR_ELSE dw.1 OR_ELSE dw.1 OR_ELSE (bEver := TRUE);

‎dw.8‎ is ‎FALSE‎ and ‎dw.1‎ is ‎TRUE‎. Therefore, the result ‎bX‎ of the operation is ‎TRUE‎. However, the expression at the third input is not executed, and ‎bEver‎ remains ‎FALSE‎. On the other hand, if the standard OR operation was used, ‎bEver‎ would be set to TRUE.