OPC UA provides a framework that can be used to represent complex information as Objectsin an AddressSpacewhich can be accessed with standard services. These Objectsconsist of Nodesconnected by References. Different classes of Nodesconvey different semantics. For example, a Variable Noderepresents a value that can be read or written. The Variable Nodehas an associated DataTypethat can define the actual value, such as a string, float, structure etc. It can also describe the Variablevalue as a variant. A Method Noderepresents a function that can be called. Every Nodehas a number of Attributesincluding a unique identifier called a NodeIdand non-localized name called as BrowseName. An Objectrepresenting a ‘Reservation’ is shown in Figure 2.
Objectand Variable Nodesrepresent instances and they always reference a TypeDefinition(ObjectTypeor VariableType) Nodewhich describes their semantics and structure. Figure 3illustrates the relationship between an instance and its TypeDefinition.
The type Nodesare templates that define all of the children that can be present in an instance of the type. In the example in Figure 3the PersonTypeObjectTypedefines two children: First Name and Last Name. All instances of PersonType are expected to have the same children with the same BrowseNames. Within a type the BrowseNamesuniquely identify the children. This means Clientapplications can be designed to search for children based on the BrowseNamesfrom the type instead of NodeIds. This eliminates the need for manual reconfiguration of systems if a Clientuses types that multiple Serversimplement.
OPC UA also supports the concept of sub-typing. This allows a modeller to take an existing type and extend it. There are rules regarding sub-typing defined in OPC 10000-3, but in general they allow the extension of a given type or the restriction of a DataType. For example, the modeller may decide that the existing ObjectTypein some cases needs an additional Variable. The modeller can create a subtype of the ObjectTypeand add the Variable. A Clientthat is expecting the parent type can treat the new type as if it was of the parent type. Regarding DataTypes, subtypes can only restrict. If a Variableis defined to have a numeric value, a subtype could restrict it to a float.
Referencesallow Nodesto be connected in ways that describe their relationships. All Referenceshave a ReferenceTypethat specifies the semantics of the relationship. Referencescan be hierarchical or non-hierarchical. Hierarchical references are used to create the structure of Objectsand Variables. Non-hierarchical are used to create arbitrary associations. Applications can define their own ReferenceTypeby creating subtypes of an existing ReferenceType. Subtypes inherit the semantics of the parent but may add additional restrictions. Figure 4depicts several References,connecting different Objects.
The figures above use a notation that was developed for the OPC UA specification. The notation is summarised in Figure 5. UML representations can also be used; however, the OPC UA notation is less ambiguous because there is a direct mapping from the elements in the figures to Nodesin the AddressSpaceof an OPC UA Server.
The OPC UA specification defines a very wide range of functionality in its basic information model. It is not expected that all Clientsor Serverssupport all functionality in the OPC UA specifications. OPC UA includes the concept of Profiles, which segment the functionality into testable certifiable units. This allows the definition of functional subsets (that are expected to be implemented) within a companion specification. The Profilesdo not restrict functionality, but generate requirements for a minimum set of functionality (see OPC 10000-7).