To access the plant data (read/write), which server is lower level, PI or OPC UA?
Plant Data -> OPC UA -> PI or
Plant Data -> PI > OPC UA or
Usually the data flow roughly:
Plant Data -> OPC UA server -> PI OPC IF (or PI OPC UA Connector lately) -> PI Server(s)
The OPC UA server can be an integrated part of the Process Control system or it can be a separate third party 'gateway' also.
You can find detailed description for better understanding:
Figures in sections 3.5 and 11.2 specially!
Hope it helps,
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