Extending a function block

The extension of a function block is based on the concept of inheritance in object-oriented programming. A derived function block "extends" a basic function block for this purpose and thus basically obtains ("inherits") the properties and functionalities of the basic function block – in addition to its own properties and functionalities.

In TwinCAT, the term "function block" can be used interchangeably with "class". A derived function block can therefore be referred to as "subclass", and the basic function block as "base class".

Observe the following information:

 

Extending a function block 1:

Number of extensions for each basic function block

The number of extensions for each basic function block is unlimited. A function block an therefore be extended and customized with several other function blocks.

  • Possible:
    FUNCTION_BLOCK FB_Sub1 EXTENDS FB_Base
    FUNCTION_BLOCK FB_Sub2 EXTENDS FB_Base
    FUNCTION_BLOCK FB_Sub3 EXTENDS FB_Base
Extending a function block 2:

Number of inheritance levels

The number of inheritance levels is unlimited. A function block that extends another function block can therefore by customized with a further function block, etc.

  • Possible:
    FUNCTION_BLOCK FB_Sub EXTENDS FB_Base
    FUNCTION_BLOCK FB_SubSub EXTENDS FB_Sub
    FUNCTION_BLOCK FB_SubSubSub EXTENDS FB_SubSub
Extending a function block 3:

Multiple inheritance is not allowed

Multiple inheritance is not allowed for function blocks. A function block cannot extend more than one other function block.

Exception: A function block can implement several interfaces and an interface can extend several other interfaces.

  • Not possible:
    FUNCTION_BLOCK FB_Sub EXTENDS FB_Base1, FB_Base2
  • Possible:
    FUNCTION_BLOCK FB_Sample IMPLEMENTS I_Sample1, I_Sample2
  • Possible:
    INTERFACE I_Sub EXTENDS I_Base_1, I_Base_2
Extending a function block 4:

Overloaded

Overloading of methods is not possible. Therefore, if you overwrite or extend a base class method (same method name) in a subclass, the method declaration must match the base class declaration (access modifier, return type, variable interface).

Extending a basic function block with a new function block

The currently open project has a basic function block, for example FB_Sample, which is to be extended with a new function block.
1. Select the PLC project object or a subfolder in the PLC project tree, and in the context select the command menu Add > POU...
The Add POU dialog opens.
2. Enter a name for the new function block in the Name input field, for example FB_SampleEx.
3. Select Function Block.
4. Select Extends and click on Extending a function block 5:.
5. In the input assistant, from the Functionblocks category under the project select the POU(FB) to be used as basic function block, for example “FB_Sample”, and click OK.
6. Optionally you can select an Access specifier for the new function block from the combo box.
7. From the Implementation language select “Structured Text (ST)”, for example.
8. Click on Open.
TwinCAT adds the function block FB_SampleEx in the PLC project tree, and the editor opens. The first line says:
FUNCTION_BLOCK FB_SampleEx EXTENDS FB_Sample
The function block FB_SampleEx extends the basic function block FB_Sample.

Extending a basic function block with an existing function block

The currently open project has a basic function block, for example “FB_Sample”, and a further function block, for example “FB_SampleEx”, which is not yet derived from the basic function block. The function block FB_SampleEx is to extend the basic function block, i.e.: FB_SampleEx is to extend FB_Sample.
1. Double-click on the function block FB_SampleEx in the PLC project tree.
The POU editor opens.
2. Extend the existing entry in the top row, FUNCTION_BLOCK FB_SampleEx, with EXTENDS FB_Sample.
The function block FB_SampleEx extends the basic function block FB_Sample.

See also:

Further Information