DataItemsare always defined as data components of other Nodes in the AddressSpace. They are never defined by themselves. A simple example of a container for DataItems would be a “Folder Object” but it can be an Objectof any other type.

Figure 4illustrates the basic AddressSpacemodel of a DataItem, in this case an AnalogItem.

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Figure 4– Representation of DataItems in the AddressSpace

Each DataItemis represented by a DataVariablewith a specific set of Attributes. The TypeDefinitionreference indicates the type of the DataItem(in this case the AnalogItemType). Additional characteristics of DataItems are defined using Properties. The VariableTypesin 5.2specify which properties may exist. These Propertieshave been found to be useful for a wide range of Data Access clients. Serversthat want to disclose similar information should use the OPC-defined Propertyrather than one that is vendor-specific.

The above figure shows only a subset of Attributes and Properties. Other Attributes that are defined for Variablesin OPC 10000-3(e.g., Description) may also be available.