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.

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.

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 OPC Classic specifications have served the OPC community well. However, as technology evolved, so did the need for OPC specifications.

In 2008, the OPC Foundation released OPC Unified Architecture (OPC UA), a platform independent service-oriented architecture that integrates all the functionality of the existing OPC Classic specifications. Several factors influenced the decision to create OPC UA:

  • Microsoft has de-emphasized COM (Component Object Model) and DCOM (Distributed COM) in favor of cross-platform SOA (Service-Oriented Architecture).
  • OPC vendors want a single set of services to expose the OPC data models, such as Data Access, Alarms & Events, Historical Data Access, etc.
  • To stay competitive, OPC vendors need to implement OPC on non-Microsoft systems, including embedded devices.
  • Other collaborating organizations need a reliable, efficient way to transport high-level structured data.
  • Users require the ability to access OPC servers through firewalls in a secure manner.

To learn more about the OPC UA specifications see this page.