When you create an OPC DA server in Citect SCADA, you can determine if it will present items to a client application as a hierarchy (representing the clusters and equipment definitions used), or as a flat structure.
The ability to use a flat structure allows the server to comply with the item browsing definitions supported by version 2.x of the OPC DA standard.
You can set the type of browsing supported by an OPC DA server via the Browsing Hierarchy field on the OPC DA Server Properties dialog. By default, the browsing hierarchy will be set to flat.
The browse position will be initially set as the root of the address space. After the initial call to browse items, the client will be able to continue browsing from the last browse point.
In the case where a SCADA project is configured with a large number of tags it is recommended to test browsing of OPC DA items to establish if excessive memory consumption occurs on IOServers or OpcDaServer processes running in the system. In general you should attempt to browse tags using third party OPC DA Client installed on the machine where the OPC DA Process is running.
Notes:
1. Data Type filtering is not supported in the browsing function. This is due to the fact that Citect SCADA data types are not known until tags are subscribed to, if the user interface and other clients are subscribed at different points in time, clients could get a different list of tags for the same filter being applied. This is an accepted relaxation of the OPC Foundation CTT rule.
2. The actual "fully qualified" subscription name for an OPC DA Item might differ to the name returned by the OPC DA Browsing APIs, which is in accordance with OPC DA v2.05 and 3.0 specification. For example, in a multiclustered system "Tag1" might be defined in several clusters so its "fully qualified" item id might be "Cluster1.Tag1". Subscription to "Tag1" might still work where there is no ambiguity. In general it is recommended that OPC DA Clients obtain "fully qualified" ITEM ID via GetItemID if OPC DA v2.05 is used, or from OPCITEMPROPERTY structure if OPC DA v3.0 is used.
See Also
Published June 2018