This annex describes the basic infrastructure to reference from an OPC UA Information Model to external dictionaries like IEC Common Data Dictionary or eCl@ss. It defines ObjectTypes and ReferenceTypes and explains how they should be used.
The ObjectTypes are used to represent an external dictionary in an OPC UA AddressSpace. The ReferenceType is used to reference from Nodes in the AddressSpace to the dictionary entries. Such dictionary entries can be seen as external classification or external semantic.
The type system of OPC UA already provides means to express the semantic of an OPC UA Object. As an example, OPC 10000-100 defines the DeviceType ObjectType expressing that instances of this ObjectType represent devices. Subtypes of the DeviceType are used to add vendor specific semantic. However, the classification and additional semantic of the device in terms of an external data dictionary is not specified further. This annex provides means to represent that an Object is for example a differential pressure transmitter in the context of an IEC Common Data Dictionary. This allows clients to automatically retrieve and identify such devices.
This annex is an integral part of this standard, that is, the types defined in this annex have to be used as defined. However, it is not required but strongly recommended that a Server uses the types defined in this annex to refer to external dictionaries. The defined types may be subtyped to refine their behaviour.
When a Server references external dictionaries using the types defined in this annex, it refers from OPC UA Nodes to dictionary entries. The Server may optionally also provide the hierarchy and content of the external dictionary. Resource consumption needs to be considered, especially in scenarios where the OPC UA Server is part of the firmware of a device.
The following clauses define the representation of dictionary references in OPC UA and give examples for the use of dictionary references.