What is OPC UA and why does it replace classic OPC DA?
Classic OPC (DA, HDA, AE) was the industrial communication standard of the 1990s, based on Windows COM/DCOM technology. It worked well for decades, but it had serious limitations: it only ran on Windows, it was complex to configure on networks with firewalls, and it had no built-in security.
OPC UA (IEC 62541) solves all of those problems. It is cross-platform, runs over standard TCP/IP (port 4840), includes native authentication and encryption, and defines structured information models — not just simple tag values.
OPC UA client-server architecture
OPC UA works on a client-server model: the OPC UA server runs on the device that holds the data (PLC, SCADA, Historian) and exposes a tree of nodes with structured information. The OPC UA client connects, browses the tree, subscribes to value changes and reads/writes data.
The key elements of the architecture are:
- Address Space — a hierarchical tree of all available objects, variables and methods
- Subscriptions — the client subscribes to value changes instead of polling constantly
- Sessions — an authenticated connection with a configurable timeout
- Security Policies — None, Basic128Rsa15, Basic256, Basic256Sha256
OPC UA vs Modbus vs MQTT
| Protocol | Security | Cross-platform | Info model | Best use |
|---|---|---|---|---|
| OPC UA | Native | Yes | Rich/structured | PLC↔SCADA↔Historian integration |
| Modbus TCP | No security | Yes | Registers only | Legacy, simple devices |
| MQTT | Optional TLS | Yes | Publish/subscribe | IoT, telemetry, cloud |
| OPC DA | No security | Windows only | Simple tags | Legacy Windows systems |
Practical implementation
In practice, the steps to integrate OPC UA into a project are:
- Enable the OPC UA server on the PLC — in Siemens TIA Portal, Allen-Bradley Studio 5000 or the manufacturer's software
- Configure the security policy — at least Basic256Sha256 in critical industrial environments
- Use an OPC UA client to validate — UaExpert (free) lets you browse the address space and verify values
- Connect the target client — Node-RED (node-opcua), Python (asyncua), Ignition, or the Historian of your choice
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