Rockwell Automation T8030S Trusted OPC Server Package - Manual (Page 13)

Trusted
TM
OPC T8030S
Issue 23 April 10 PD-T8030S 13
1.2.4. OPC Data Access vs. OPC Alarm & Event
OPC Data Access and OPC Alarm Event clients are based on separate OPC standards. While some
OPC clients support both standards within the same package, most OPC clients will support one or the
other. OPC Data Access clients are the most common, and are used by many MMI (Man Machine
Interface) packages to monitor specific process variables originating from a Trusted
TM system. OPC
Alarm & Event clients are used primarily in “Event Historian” or “Event Log” type applications.
OPC Data Access clients query the server by tag name. They can query the value of a specific tag,
create groups of tags which will be updated at regular intervals, subscribe to a tag to receive updates
when that tag’s value changes, or change the value of the tag in the Trusted
TM controller. The server
allows Data Access clients to access any Controller tag that either has a communications address or is
configured for Sequence of Event or Process Historian updates.
OPC Alarm & Event clients query the server by controller name. Instead of being able to query by tag
name, the Alarm & Event clients receive all events originating from subscribed controllers. The server
will generate events only for tags defined by a Controller that are configured for Sequence of Event
updates.
1.2.5. Communications Interface
The server manages all OPC client queries, updates, polling groups, and subscriptions. The server
translates these requests into a combination of both polled and event updates, which are sent to the
appropriate Trusted
TM controller. As is the intention of the OPC standards, the OPC clients do not
know about or need to understand the specific communication details between the server and the
Controllers.