OPC DataAccess Server is comprised of several objects: the server, the group, and
the item The OPC server object maintains information about the server and serves as a container for OPC group objects. The OPC group object maintains information about itself and provides the
mechanism for containing and logically organizing OPC items.
Data can be read and written
Exception based connections can also be created between the client and the items in the group and can be enabled and disabled as needed.
There are two types of groups, public and local (or ‘private’).Public is for sharing across multiple
clients, local is local to a client
The OPC Items represent connections to data sources within the server
An OPC Item, from the custom interface perspective, is not accessible as an object by an OPC Client All access to OPC Items is via an OPC Group object that “contains” the OPC item
Note that the items are not the data sources - they are just connections to them.
For example, the tags in a DCS system exist regardless of whether an OPC client is currently accessing them. The OPC Item should be thought of as simply specifying the address of the data, not as the actual physical source of the data that the address references.
An OPC client application communicates to an OPC server through the specified custom and automation interfaces. OPC servers must implement the custom interface, and optionally may implement
the automation interface
