Overview of the Custom Interface

Overview of the OPC Custom Interface The OPC Custom Interface Objects include the following custom objects:

OPCServer
OPCGroup

The interfaces and behaviors of these objects are described in detail in this chapter. Developers of OPC servers are required to implement the OPC objects by providing the functionality defined in this chapter.

This chapter also references and defines expected behavior for the standard OLE interfaces. Interfaces that an OPC server and an OPC client are required to implement when building OPC compliant components.

Also, standard and custom Enumerator objects are created, and interfaces to these objects are returned. In general the enumerator objects and interfaces are described briefly since their behavior is well defined by OLE.

The OPC specification follows the preferred approach that enumerators are created and returned from methods on objects rather than through QueryInterface. The enumerators are as follows:

Group Enumerator - (see IOPCServer::CreateGroupEnumerator)
Item Attribute Enumerator - (see IOPCItemMgt::CreateEnumerator)
Server Address Space Enumerator - (see IOPCBrowseServerAddressSpace::BrowseOPCItemIDs)
AccessPath Enumerator - (see IOPCBrowseServerAddressSpace::BrowseAccessPaths)

Also you will note that in some cases lists of things are returned via enumerators and in other cases as simple lists of items. Our choice depends on the expected number of items returned. ‘Large’ lists are best returned through enumerators while ‘small’ lists are more easily and efficiently returned via explicit lists.