The abbreviation OPC Classic (OLE for Process Control) refers to an interface
which is defined and expanded by the OPC Foundation. OPC Classic
is widely used in process automation, in particular for coupling process data to
HMI systems.
OPC Classic is based on a client-server structure. A client (master) can access
one or several servers (slaves). Several clients can simultaneously access one
server. One server provides the data and the client receives the data from the
server. Although the client can also transmit data to the server, the main direction of
data flow is from the server to the client.
OPC Classic comprises a number of interfaces which serve various purposes.
The most important are:
- DA (Data Access)
this is probably the most widely used interface defining interfaces and
methods for accessing process data
- AE (Alarm and Event)
is an add-on to the DA interface and enables the transmission of events and
alarms
- HD (Historical Data)
is an add-on to the DA interface and enables the transmission of historical
data
- DA XML
This interface is a fairly recent addition based on the DA interface. Data
encoding is however based on XML (eXtended Markup
Language).
Unlike other protocols which have mainly been developed for the transmission
of process data, the OPC Foundation
allows users to verify the compliance of the server implementation with the
standard. This is done with the Compliance Test Tool (CTT), a software package
which is made available to members free of charge. If the server passes the
compliance test, a document is generated as compatibility proof and functional
reference. The CCT enables the user to virtually eliminate incompatibilities
between implementations from different manufacturers and ensure smooth
integration.
In addition, workshops are organized on a regular basis, where different
manufacturers can benchmark their systems against each other.
