The instantiation of complex TypeDefinitionNodes depends on the ModellingRules defined in 6.4.4. However, the intention is that instances of a type definition will reflect the structure defined by the TypeDefinitionNode. Figure 7 shows an instance of the TypeDefinitionNode “AI_BLK_TYPE”, where the ModellingRule Mandatory, defined in 18.104.22.168.2, was applied for its containing Variable. Thus, an instance of “AI_BLK_TYPE”, called AI_BLK_1”, has a HasTypeDefinition Reference to “AI_BLK_TYPE”. It also contains a Variable “SP” having the same BrowseName as the Variable “SP” used by the TypeDefinitionNode and thereby reflects the structure defined by the TypeDefinitionNode.
Figure 7 – Object and its Components defined by an ObjectType
A client knowing the ObjectType “AI_BLK_TYPE” can use this knowledge to directly browse to the containing Nodes for each instance of this type. This allows programming against the TypeDefinitionNode. For example, a graphical element may be programmed in the client that handles all instances of “AI_BLK_TYPE” in the same way by showing the value of “SP”.
There are several constraints related to programming against the TypeDefinitionNode. A TypeDefinitionNode or an InstanceDeclaration shall never reference two Nodes having the same BrowseName using forward hierarchical References. Instances based on InstanceDeclarations shall always keep the same BrowseName as the InstanceDeclaration they are derived from. A special Service defined in OPC 10000-4 called TranslateBrowsePathsToNodeIds may be used to identify the instances based on the InstanceDeclarations. Using the simple Browse Service might not be sufficient since the uniqueness of the BrowseName is only required for TypeDefinitionNodes and InstanceDeclarations, not for other instances. Thus, “AI_BLK_1” may have another Variable with the BrowseName “SP”, although this one would not be derived from an InstanceDeclaration of the TypeDefinitionNode.
Instances derived from an InstanceDeclaration shall be of the same TypeDefinitionNode or a subtype of this TypeDefinitionNode.
A TypeDefinitionNode and its InstanceDeclarations shall always reside in the same Server. However, instances may point with their HasTypeDefinition Reference to a TypeDefinitionNode in a different Server.