DataItems are 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 Object of any other type.
Figure 4 illustrates the basic AddressSpace model of a DataItem, in this case an AnalogItem.
Figure 4 – Representation of DataItems in the AddressSpace
Each DataItem is represented by a DataVariable with a specific set of Attributes. The TypeDefinition reference indicates the type of the DataItem (in this case the AnalogItemType). Additional characteristics of DataItems are defined using Properties. The VariableTypes in 5.2 specify which properties may exist. These Properties have been found to be useful for a wide range of Data Access clients. Servers that want to disclose similar information should use the OPC-defined Property rather than one that is vendor-specific.
The above figure shows only a subset of Attributes and Properties. Other Attributes that are defined for Variables in OPC 10000-3 (e.g., Description) may also be available.