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 type of 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 NodeIdand a non-localized name called BrowseName. An Objectrepresenting a ‘Reservation’ is shown in Figure 4.

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Figure 4– A Basic Object in an OPC UA Address Space

Objectand Variable Nodesrepresent instances and they always reference a TypeDefinition(ObjectTypeor VariableType) Nodewhich describes their semantics and structure. Figure 5illustrates 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 5the 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 subtyping. This allows a modeller to take an existing type and extend it. There are rules regarding subtyping 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 sub type could restrict it to a float.

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Figure 5– The Relationship between Type Definitions and Instances

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 6depicts several References,connecting different Objects.

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Figure 6– Examples of References between Objects

The figures above use a notation that was developed for the OPC UA specification. The notation is summarized in Figure 7. 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.

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Figure 7– The OPC UA Information Model Notation

A complete description of the different types of Nodesand Referencescan be found in OPC 10000-3and the base structure is described in OPC 10000-5.

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 functionalities (see OPC 10000-7).

OPC UA allows information from many different sources to be combined into a single coherent AddressSpace. Namespaces are used to make this possible by eliminating naming and ID conflicts between information from different sources. Namespaces in OPC UA have a globally unique string called a NamespaceUri and a locally unique integer called a NamespaceIndex. The NamespaceIndex is only unique within the context of a Sessionbetween an OPC UA Clientand an OPC UA Server. The Servicesdefined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.

There are two types of values in OPC UA that are qualified with Namespaces: NodeIds and QualifiedNames. NodeIds are globally unique identifiers for Nodes. This means the same Nodewith the same NodeId can appear in many Servers. This, in turn, means Clients can have built-in knowledge of some Nodes. OPC UA Information Modelsgenerally define globally unique NodeIdsfor the TypeDefinitionsdefined by the Information Model.

QualifiedNames are non-localized names qualified with a Namespace. They are used for the BrowseNamesof Nodesand allow the same names to be used by different information models without conflict. TypeDefinitionsare not allowed to have children with duplicate BrowseNames; however, instances do not have that restriction.

An OPC UA companion specification for an industry specific vertical market describes an Information Modelby defining ObjectTypes, VariableTypes, DataTypesand ReferenceTypesthat represent the concepts used in the vertical market, and potentially also well-defined Objects as entry points into the AddressSpace.