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1 Overview
The OPC Foundation is a cooperation of automation control product vendors and Microsoft.
As of 2006, the foundation had about 300 members worldwide. The main standard, with
which it all began, is the OPC DA (Data Access) specification.
Compliance tests are provided to the members to test OPC conformity.
1.2.2
COM
The COM (Component Object Model) interface defines the externally visible binary structure
of a COM object. The COM object is hereby the implementation of this model and can be
based on several interfaces which define access to the object. It offers runtime access to
the incorporated interfaces of a COM object and interprocess communication. Reference
counting is implemented to manage the object's life cycle. Interactions between components
are based on method-accesses defined in interfaces. COM applications are restricted to run
locally on single computers only.
1.2.3
DCOM
Upon inception, DCOM (Distributed Component Object Model) faced two major challenges:
•
program-based processing of information, which led to information being represented
differently
•
backwards compatibility to existing COM and DDE applications
OLE is based directly on DCOM. The DCOM interface allows COM applications to be
distributed on several computers, providing a single security concept.
DCOM applications are restricted by not:
•
running over firewalls
•
being platform independent (only Microsoft operating systems are supported)
1.2.4
OPC Data Access (DA)
Version 1 of the OPC DA (Data Access) standard was released in 1996. Today version 3.0
is available. A task force within the
specification.
The specification shows and describes how to construct client and server applications
and allows vendors to quickly develop their own applications. It defines object hierarchy
consisting of several OPC server objects, which in turn can consist of several OPC group
objects. The group objects themselves can be composed of OPC item objects. OPC items
correspond to process values and represent connections to data sources.
12
OPC Foundation
maintains and revises the OPC DA
ACGM0143V4EN
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