OPC is the interoperability layer of industrial automation - the set of standards that let SCADA, HMI, and historian software exchange data with controllers and devices regardless of vendor. Understanding OPC means understanding two generations: OPC Classic, built on Windows COM/DCOM, and OPC UA, the modern, platform-independent successor.
OPC in one line: OPC is a family of interoperability standards from the OPC Foundation; OPC Classic (DA, HDA, A&E) relied on Windows COM/DCOM to move real-time, historical, and alarm data, while OPC UA replaces it with a secure, platform-independent, service-oriented architecture and a rich information model.
OPC originally stood for OLE for Process Control and was built on Microsoft's COM/DCOM technology. It defined several specifications for different data types: OPC DA (Data Access) for real-time process values, OPC HDA (Historical Data Access) for stored history, and OPC A&E (Alarms and Events) for alarm streams. An OPC server (often shipped by a device or driver vendor) exposed device data, and OPC clients such as SCADA and historians consumed it - a single translation layer instead of a bespoke driver per product.
The weakness was DCOM. Because OPC Classic depended on Windows-only DCOM for communication between machines, it was difficult to configure across firewalls, painful to secure, and effectively tied to Windows. Connecting an OPC client and server on different computers - especially across network boundaries - became a recurring source of frustration for integrators, which is precisely the problem the next generation set out to solve.
OPC UA (Unified Architecture) is a ground-up redesign. It drops COM/DCOM in favour of its own service-oriented transport that runs on any operating system and traverses firewalls cleanly, with security (authentication, encryption, and signing) built into the specification rather than bolted on. It unifies the old DA, HDA, and A&E capabilities into one framework, so a single UA connection can carry current values, history, and events.
Just as important is the OPC UA information model. Instead of a flat list of tags, UA lets a server expose a structured, self-describing address space where objects, variables, methods, and their relationships are modelled - so a client can browse and understand an asset, not just read numbers. This makes UA well suited to modern, multi-site, and cloud architectures. A cloud SCADA such as Merobix connects to controllers and OPC UA servers over OPC UA, reading tags securely and pushing them outbound to cloud dashboards, alarms, and a historian, which is why UA has become a default choice for new integrations.
OPC Classic (DA/HDA/A&E) is built on Windows COM/DCOM, which ties it to Windows and makes cross-machine and cross-firewall connections difficult. OPC UA is platform-independent, uses its own secure service-oriented transport that traverses firewalls, unifies real-time/historical/alarm data, and adds a rich, browsable information model.
Originally, yes - OPC stood for OLE for Process Control, reflecting its COM/OLE roots. With OPC UA, the OPC Foundation moved to the platform-independent Unified Architecture, and OPC is now treated as the name of the standard rather than the old acronym.
For new projects, largely yes. OPC UA is the strategic direction because it is secure, cross-platform, and better suited to remote and cloud connectivity. Many installed systems still use OPC Classic, and COM/DCOM-to-UA wrappers or gateways let those systems bridge into UA-based architectures.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.