OPC Classic

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.