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7.4 Node classes
OPC nodes have an inheritance model, based on their
NodeClass
.
There are eight base node classes defined by the standard:
• Object
• Variable
• Method
• View
• ObjectType
• VariableType
• ReferenceType
• DataType
All nodes have the same base Attributes (inherited from the Node object), plus additional ones depending on their
NodeClass
.
7.5 Node graph and references
It may appear that nodes are only chained hierarchically, in a simple parent-child relationship. However, in reality nodes are
chained in a complex directed graph, through
References
to other nodes.
Figure 24. Hierarchy of the standard ReferenceTypes, defined in Part 3 of the OPC UA specification (Image taken from
www.open62541.org)
In OPC, even ReferenceTypes are Nodes, and as such are structured hierarchically, as can be seen in the figure above.
The definitions of all OPC ReferenceTypes can be found under the
0:Root,0:Types,0:ReferenceTypes
path.
The semantics of OPC references can be enriched by creating custom ReferenceType nodes.
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