OPC Classic

Hardware <------> PLC <------> (DCOM)Server <------> Client

The OPC Classic specifications are based on Microsoft Windows technology using the COM/DCOM (Distributed Component Object Model) for the exchange of data between software components. The specifications provide separate definitions for accessing process data, alarms and historical data.

OPC Data Access (OPC DA) The OPC DA Specification defines the exchange of data including values, time and quality information. The most basic protocol of the OPC stack is the Data Access protocol that gets data out of the control systems into other systems on the shop floor. Each information about a specific tag or data point contains some information about it. First you have the data itself and that is called Value and of course the Name of it. To that comes a number of other pieces of information that describes the information, the first is the Timestamp that gives you the exact time when the value was read. This timestamp can be taken either directly from the underlying system or assigned to it when the data is read in the OPC server. The last piece is called Quality which gives a basic understanding if the data is valid or not.

OPC Alarms & Events (OPC AE) The OPC A&E Specification defines the exchange of alarm and event type message information, as well as variable states and state management. The second protocol to be added to the OPC stack was Alarms & Events. This protocol is fundamentally different from the DA protocol simply due to the fact that events not have a current value. This means that this protocol always is a subscription based service where the clients gets all the events that comes in. In terms of data that comes with the event there is no tags and therefore not any name and quality but there is of course a Timestamp. But like in the case with DA there is no store in the server and once the event is transferred the server forgets it was ever there.

OPC Historical Data Access (OPC HDA) The OPC HDA Specification defines query methods and analytics that may be applied to historical, time-stamped data. The difference between DA, AE and HDA is that HDA contains historical data and you can call for a large amount of past data. The protocol therefore supports long record sets of data for one or more data points. It was designed to provide a unified way to get out and distribute historical data stored in SCADA or Historian systems like OSI-PI or Historian from GE. The protocol is not so widely used today and now the introduction of OPC UA makes it somewhat obsolete.

OPC Classic continues to be an integral part of the OPC technology portfolio. In 2010, OPC Classic was enhanced with the OPC .NET 4.0 specifications to adapt to the new technical innovations of the Microsoft platforms providing better connectivity, reliability, security and interoperability.