UK What OPC UA is
OPC UA (OPC Unified Architecture) is an open, platform-independent standard for exchanging data between industrial machines, controllers and software. It is published by the OPC Foundation and standardised internationally as IEC 62541. OPC originally stood for OLE for Process Control and now stands for Open Platform Communications.
OPC UA does more than move numbers. Each server publishes an information model: a structured, browsable address space that describes what a value is, its engineering range, whether it is in bar, °C or rpm, and how it relates to the rest of the machine. A client can connect to an unfamiliar device, browse its model and understand the data without a separate register map, which is the main difference from simpler protocols such as Modbus.
The first OPC UA specifications were released in 2008 to replace the older Windows-only OPC Classic standards. It now runs on PLCs, sensors, edge computers, SCADA and MES software, and cloud platforms.
How OPC UA works
OPC UA defines the data model, the services that act on it, the security and the way messages are encoded and carried. Applications take one or both of two roles.
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Server. Runs on or next to the equipment, often embedded in a PLC or drive, and exposes its data as nodes in an address space: variables, methods, events and alarms.
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Client. An HMI, SCADA system, historian, MES or analytics application that connects to servers to read, write, subscribe to changes and call methods.
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Subscriptions. Instead of polling, a client asks the server to report a value only when it changes by more than a set amount, which keeps network traffic low.
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PubSub. Added in 2018, publish-subscribe lets one publisher send data to many subscribers over UDP multicast or through a broker using MQTT or AMQP, without a session for each client.
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Transports. The usual binary protocol runs over TCP and uses port 4840 by default (opc.tcp://). HTTPS and WebSockets are also defined for web and firewall-friendly use.
OPC UA vs OPC Classic
Many plants still run OPC Classic, the earlier family of specifications built on Microsoft COM and DCOM. OPC UA keeps their functions but removes the dependence on Windows.
| OPC Classic (DA, HDA, A&E) | OPC UA | |
|---|---|---|
| Platform | Windows only, built on COM/DCOM | Any operating system, including Linux and embedded devices |
| Scope | Separate specs for live data, history and alarms | One unified model covering all three, plus methods |
| Data model | Flat list of tags | Object-based, typed and browsable information model |
| Security | Relies on Windows and DCOM settings | Built in: X.509 certificates, signing and encryption |
| Firewalls | DCOM is hard to route through firewalls | A single configurable TCP port, 4840 by default |
| Standard | OPC Foundation specifications | OPC Foundation, also IEC 62541 |
Gateways and wrappers can bridge the two, so a legacy OPC DA server can be read by an OPC UA client during a migration.
OPC UA security
Security was designed into OPC UA from the start, which is one reason it is widely used across the boundary between IT and OT networks.
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Application authentication. Every client and server has an X.509 certificate, and each side decides which certificates to trust.
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Message security. Connections can be set to None, Sign or SignAndEncrypt. Production systems should use SignAndEncrypt with a current security policy.
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User authentication. Anonymous access, user name and password, or user certificates, with access rights set per node.
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Auditing. Servers can log security events, which supports IEC 62443 practice for industrial control systems.
Companion specifications and where OPC UA runs
On top of the base standard, industry groups publish companion specifications that define standard information models for a type of equipment, such as injection moulding machines, robotics, machine tools, packaging lines and process instruments. Two machines that follow the same companion specification present their data the same way, which cuts integration work. Dozens are published, many developed jointly with groups such as VDMA, PLCopen and ISA.
OPC UA servers are now built into many PLCs, drives and sensors. Where they are not, an edge computer runs the server or a protocol converter. Our industrial IoT gateways and DIN rail PCs include Arm and x86 systems whose catalogue descriptions list protocol conversion, data logging and MQTT or OPC UA gateway roles. For deterministic control traffic, work is under way on OPC UA FX over time-sensitive networking (TSN).
For a side-by-side comparison of the two protocols, including Sparkplug B and gateway designs, see our OPC UA vs MQTT guide.
Quick answers on OPC UA
OPC UA: common questions
What does OPC UA stand for?
Open Platform Communications Unified Architecture. OPC first stood for OLE for Process Control, after the Microsoft technology the original standards used. “Unified” refers to merging live data, history and alarms into one model.
Is OPC UA a protocol?
It is a framework that includes protocols. OPC UA defines an information model, services and security as well as the wire protocols (UA Binary over TCP, HTTPS, WebSockets and PubSub) that carry them.
What port does OPC UA use?
TCP 4840 by default. Port 4840 is registered for OPC UA and is used by discovery servers and many devices. Servers can be set to other ports, so check the endpoint URL.
What is the difference between OPC UA and MQTT?
OPC UA models data; MQTT only moves it. MQTT is a lightweight publish-subscribe transport with no built-in data model. OPC UA describes what the data means and can itself publish over MQTT. Many systems use OPC UA on the plant floor and MQTT to the cloud.
Is OPC UA free to use?
The specifications are open. Anyone can read and implement them, and open-source stacks exist. Membership of the OPC Foundation brings extra resources, and certified products go through the Foundation’s compliance testing.
We supply edge computers and gateways with the operating system, drivers and network interfaces configured as standard, ready for your OPC UA server or client software.