Specifications

RSLinx – Training Guide B - 15
related to specific conditions. OPC Clients may subscribe to be notified of the occurrence
of specified events.
The IOPCEventServer interface provides methods enabling the OPC Client to:
• Determine the types of events which the OPC Server supports.
• Enter subscriptions to specified events, so that OPC Clients can receive notifications of their
occurrences. Filters may be used to define a subset of desired events.
• Access and manipulate conditions implemented by the OPC Server.
In addition to the IOPCEventServer interface, an OPC Event Server may support optional
interfaces for browsing conditions implemented by the server and for managing public
condition groups (defined in the following section).
OPC Historical Data Access Overview
Historical engines today produce an added source of information that must be distributed to
users and software clients that are interested in this information. Currently most historical
systems use their own proprietary interfaces for dissemination of data. There is no
capability to augment or use existing historical solutions with other capabilities in a plug-n-
play environment. This requires the developer to recreate the same infrastructure for their
products as all other vendors have had to develop independently with no interoperability
with any other systems.
In keeping with the desire to integrate data at all levels of a business, historical information
can be considered to be another type of data.
There are several types of Historian servers. Some key types supported by this
specification are:
• Simple Trend data servers. These servers provided little else then simple raw data
storage. (Data would typically be the types of data available from an OPC Data Access
server, usually provided in the form of a tuple [Time Value & Quality])
• Complex data compression and analysis servers. These servers provide data
compression as well as raw data storage. They are capable of providing summary data
or data analysis functions, such as average values, minimums and maximums etc. They
can support data updates and history of the updates. They can support storage of
annotations along with the actual historical data storage.
Where OPC Fits
Although OPC is primarily designed for accessing data from a networked server, OPC
interfaces can be used in many places within an application. At the lowest level they can get
raw data from the physical devices into a SCADA or DCS, or from the SCADA or DCS
system into the application.. The architecture and design makes it possible to construct an
OPC Server which allows a client application to access data from many OPC Servers
provided by many different OPC vendors running on different nodes via a single object.