Extension of a Function Block

Overview

Supporting object-orientated programming, a function block can be derived from another function block. This means a function block can extend another, thus automatically getting the methods/properties/actions/transitions and variables of the basing function block in addition to its own.

The extension is performed by using the keyword EXTENDS in the declaration of a function block. You can choose the EXTENDS option already during adding a function block to the project via the Add Object dialog box.

For further information, refer to the sections describing how to add a method, a property, an action, or a transition.

Syntax

FUNCTION_BLOCK <function block name> EXTENDS <function block name>

This is followed by the declaration of the variables.

Example

Definition of function block fbA

FUNCTION_BLOCK fbA
VAR_INPUT
    x:int;
END_VAR
...

Definition of function block fbB

FUNCTION_BLOCK fbB EXTENDS fbA
VAR_INPUT
    ivar: INT := 0;
END_VAR
...

Extension by EXTENDS

Extension by EXTENDS means:

• fbB contains all data and methods which are defined by fbA. An instance of fbB can now be used in any context where a function block of type fbA is expected.

• fbB is allowed to override the methods/properties/actions/transitions defined in fbA. This means: fbB can declare a method with the same name and the same inputs and output as declared by A.

• fbB is not allowed to declare function block variables with the same name as used in fbA. In this case, the compiler will generate an error message.

• fbA variables and methods can be accessed directly within an fbB scope by using the SUPER pointer (SUPER^.<method>).

Example

FUNCTION_BLOCK FB_Base
VAR_INPUT
END_VAR
VAR_OUTPUT
    iCnt : INT;
END_VAR
VAR
END_VAR
THIS^.METH_DoIt();
THIS^.METH_DoAlso();
    
    METHOD METH_DoIt : BOOL
    VAR
    END_VAR
    iCnt := -1;
    METH_DoIt := TRUE;
    
    METHOD METH_DoAlso : BOOL
    VAR
    END_VAR
    METH_DoAlso := TRUE;

FUNCTION_BLOCK FB_1 EXTENDS FB_Base
VAR_INPUT
END_VAR
VAR_OUTPUT
END_VAR
VAR
END_VAR
// Calls the method defined under FB_1
THIS^.METH_DoIt();
THIS^.METH_DoAlso();
// Calls the method defined under FB_Base
SUPER^.METH_DoIt();
SUPER^.METH_DoAlso();
    METHOD METH_DoIt : BOOL
    VAR
    END_VAR
    iCnt := 1111;
    METH_DoIt := TRUE;
PROGRAM PLC_PRG
VAR
    Myfb_1: FB_1;
    iFB: INT;
    iBase: INT;
END_VAR
Myfb_1();
iBase := Myfb_1.iCnt_Base;
iFB := Myfb_1.iCnt_THIS;