Implementing Interfaces

Implementing interfaces is based on the concept of object-oriented programming. With common interfaces, you can use different but similar function blocks the same way.

A function block that implements an interface has to include all methods and attributes that are defined in that interface (interface methods and interface attributes). This means that the name and the inputs and outputs of the methods or attributes must be exactly the same. When you create a new function block that implements an interface, CODESYS adds all methods and attributes of the interface automatically to the tree below the new function block.

Attention

If you add more interface methods afterwards, then CODESYS does not add these methods automatically to the affected function block. To perform this update, you must execute the Implement Interfaces command explicitly.

For inherited function blocks, you have to make sure that any methods or attributes that were derived through the inheritance of an interface also receive the appropriate implementation. Otherwise they should be deleted in case the implementation that was provided in the basis should be used. Respective compile error messages or warnings are displayed, prompted automatically by added pragma attributes. For more information, refer to the help page for the Implementing Interfaces command.

Attention

• You must assign the interface of a function block to a variable of the interface type before a method can be called via the variable.
• A variable of the interface type is always a reference of the assigned function block instance.

A variable of the interface type is a reference to instances of function blocks. This kind of variable can refer to every function block that implements the interface. If there is no assignment to a variable, then the variable in online mode contains the value 0.

Examples

The I1 interface contains the GetName method.

METHOD GetName : STRING

The functions blocks A and B implements the interface I1:

FUNCTION_BLOCK A IMPLEMENTS I1
FUNCTION_BLOCK B IMPLEMENTS I1

For this reason, both function blocks must include a method named GetName and the return type STRING. Otherwise the compiler reports an error.

A function includes the declaration of a variable of interface I1 type.

FUNCTION DeliverName : STRING
VAR_INPUT
    l_i : I1;
END_VAR

Function blocks that implement the I1 interface can be assigned to these input variables.

Examples of function calls:

DeliverName(l_i := A_instance); // call with instance of type A
DeliverName(l_i := B_instance); // call with instance of type B

Calling of interface methods:

In this case, it depends on the actual type of l_i whether the application calls A.GetName or B.GetName.

DeliverName := l_i.GetName();

• Command ‘Implement interfaces’

Implementing an interface in a new function block

1. Right-click Application in the device tree and select Project ‣ Add Object ‣ POU.

   ⇒

   The Add POU dialog box opens.

2. Type the name for the new POU in the Name input field, for example POU_Im.

3. Select Function block.

4. Click Implemented and then the more button ( ).

5. In the input assistant, select the interface from the category Interfaces, for example ITF1, and click on OK.

6. To insert more interfaces, click and select a another interface.

7. As an option, you can select an Access modifier for the new function block from the selection list.

8. Select from the Implementation language combo box (example: Structured text (ST).

9. Click Add.

   ⇒

   CODESYS adds the POU_Ex function block to the device tree and opens the editor. The first line contains the text:

   FUNCTION_BLOCK POU_Im IMPLEMENTS ITF1

   The interface and its methods and properties are now inserted below the function block in the device tree. Now you can type program code into the implementation part of the interface and its methods.

Implementing an interface in an existing function block

1. Double-click the POU_Ex(FB) POU in the device tree.

   ⇒

   The POU editor opens.

2. Extend the existing entry in the uppermost line FUNCTION_BLOCK POU_Im with IMPLEMENTS ITF1

   ⇒

   The POU_Im function block implements the ITF1 interface.

See also

• Object ‘Function Block’