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OPC Browse Service

Browsing elements117 of an OPC XML-DA compliant server is implemented in a hierarchical fashion. The Browse Service, as defined in [OPCXMLDA], specifies a starting element, from which browsing starts. The OPC Servers then lists items of the same hierarchy level118 and may implement the following features for limiting browse results and returning extended information about OPC elements:

Return Properties:
The client may request item properties including their values which have then to be returned by the server.
IsItem/HasChildren:
The server has to outline if the listed element is an OPC item or only a logical branch. Moreover, the server has to denote if there are elements in the next lower hierarchy level, in other words if the current element is a parent of other elements.
Filtering:
The client may specify filter expressions which can be used to exclude elements the client is not interested in.
Limiting Browse Results:
A client may also specify the maximum elements the server returns in a browsing response119.

Browsing the hierarchical tree of the IGUANA gateway cannot be mapped directly to one single ESD command as every level represents a very different dataset and is implemented in a different way. For instance, browsing on the first level will list the ``FANTypes'' which has to be implemented by querying the IGUANA gateway configuration, while browsing on the third level will list the datapoints of a node which has to be implemented entirely different.

The hierarchical tree of the IGUANA gateway has four levels plus an additional level for the encoding style. Figure 56 depicts how browsing in different hierarchy levels may be implemented in the IGUANA Gateway.

Figure 56: Possible implementation of an OPC XML-DA compliant browse service on the IGUANA Gateway
\begin{figure}\centering
\includegraphics[scale=0.7]{graphics/opc-browsing.eps}\end{figure}

Figure 56 shows that a simple OPC browse request may trigger various ESD services which may lead to high fieldbus loads and long response times. For instance, browsing one FAN with 100 fieldbus nodes will lead to one ESD NODELIST and 100 ESD NODEINFO commands which produce a high load on the fieldbus. The ESD daemon of the IGUANA gateway implements caching of certain data, which may greatly reduce the load on the fieldbus120. However, it may be appropriate to implement OPC XML-DA specific caching strategies in the OPC server itself to further reduce fieldbus load and improve the responsiveness of the OPC Server.

Extended browse features, such as filtering or limiting browse results, are not supported by ESD and have to be implemented by the OPC server.


next up previous contents
Next: OPC Subscription Service Up: Representing the IGUANA Interface Previous: OPC Status Service   Contents
Hermann Himmelbauer 2006-09-27