OPC UA

OPC UA defines an information model and a secure client-server transport that runs over Ethernet without depending on any single PLC brand. Most modern robot controllers offer an OPC UA server option that publishes axis positions, alarms, program status, and cycle data as structured nodes in a browsable address space.

In a cell, OPC UA is usually the northbound link (up to MES, historian, and analytics) rather than the horizontal link across to the line PLC. A SCADA client like Ignition subscribes to the nodes it needs and gets change notifications, instead of polling raw registers on a fixed cycle.

Because the model is self-describing, the client can browse the address space and pick tags at configuration time. That takes most of the custom register-map work out of an integration compared to Modbus TCP or a raw socket connection.

Pick OPC UA when the requirement is data collection, reporting, or MES integration. For time-critical motion or cyclic I/O exchange, look elsewhere: even with the PubSub extension the overhead is higher than a real-time fieldbus like EtherCAT or PROFINET RT.

The security model is a strength and an install-time cost. Certificate exchange, user authentication, and endpoint policy setup take real configuration effort, and skipping them defeats the point. Trust list mistakes are one of the more common reasons clients can't connect on day one.

Server implementations vary in what they expose. FANUC, KUKA, ABB, and Universal Robots each ship a different node structure, and the same tag (say, current program name) sits in a different place on each. Any client that has to span brands ends up with a translation layer.

Typical uses: pulling cycle time and defect counts from an arc welding cell into a plant dashboard, feeding a digital twin with live joint positions, or exposing job status to a MES that tracks work orders across many cells.