OPC does not represent a network protocol in the traditional sense, but rather a capability to support the interfacing and interconnection with disparate vendor technologies.
OPC is a set of several specifications for sharing data based on Microsoft technologies COM, DCOM, OLE, and RPC. Microsoft has since replaced these technologies with .NET and no longer supports these legacy technologies. OPC standards based on COM and DCOM are referred to by the OPC foundation as OPC Classic Specifications.
The OPC Data Access (DA), the most basic of the protocols and is the original of the OPC Classic Specs, defines the exchange of data including values, time, and quality information. The second protocol to be added, Alarm and Events (A&E), defines the exchange of alarm and event message information. It is a subscription service where the client recieves all incoming events. The OPC Historical Data Access (HDA) specification defines query methods and analytics that may be applied to historical, time-stamped data, and supports record data sets for one or more points.
Copyright: Cybersecurity & Infrastructure Security Agency (CISA)