OPC UA Server - Methods

The WinCC OA OPC UA server supports OPC UA methods. A method that can be called by clients is added under a defined parent node. The actions to perform within WinCC OA are defined by using Control.

Such methods can have input and output parameters. The supported parameter types are the data types supported by OPC OA (see Mapping of WinCC OA <-> OPC UA Data Types).

OPC UA methods can only be added to parent nodes of type FolderType or BaseObjectType. If methods are added to a parent node with a different type, the errors BadNodeIdUnknown or BadMethodInvalid are returned during runtime.

Functionality

The Control library wincc_oa_path/scripts/libs/opcuaSrv_Methods.ctl must be reconfigured so that the function getMethodsForOpcUaSrv(int managerNumber) returns a list of the available methods for the OPC UA server instance.

The function returns a list of UaMethod objects. To implement custom methods, a new class must be created that is derived from UaMethod.

The OPC UA server is loading the methods at start-up. Therefore, a restart is always required, if the library "opcuaSrv_Methods.ctl" or the methods were changed, for the changes to take effect.

Method Class

Example

For an example, see wincc_oa_path/scripts/examples/libs/opcuaSrv_Methods.ctl. Detailed code information can directly be found within the file wincc_oa_path/scripts/libs/opcuaSrv_Methods.ctl.

Constructor

The class derived from UaMethod must contain a constructor in which the attributes of the class must be defined. This defines, amongst other things, where in the OPC UA address space the function should be attached and which input and output parameters are required.

`Execute` Function

In addition, the execute function of the base class must be overloaded.

This function is called, when the OPC UA method is executed. The function receives the value for the previously defined input parameter and a list of output parameters as reference values.

When executing the function, the output parameter list must be filled with values and an OPC UA status code must be returned that matches the return value of the method.

The values of the output parameter must match the datatypes assigned within the constructor. The same number of output arguments must be returned as defined within the constructor, otherwise the method call will fail with the return code BadInternalError.

Error Analysis

To discover potential error sources the debug flag

-dbg 27

respectively -dbg DRV_USR3 can be used. The flag will return detailed information about which methods were loaded at manager start. In addition, further information is provided during the execution of OPC UA methods.