OPC instead of modbus
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As I understand it, you are opting to switch from Modbus RTU to Modbus Ethernet? (Otherwise I can't see major benefits)
The ABB 800xA system supports redundant Connectivity, like PLCConnect (?) for supporting the object-oriented model, which is a fundamental part of the system.
To answer the question of redundancy is possible to use? I would say it is up to the OPC vendor and the ESD (HIMA) system.
From the licensing point of view you need to license your 800xA with new connectivity (PLCConnect) if you don't have it, or if you have just a couple of signals use generic OPC (no license). I would strongly recommend the first alternative.
Of course you need a third-party license for the OPC-server (Modbus Ethernet) like the Kepware EX.
Depending on the size of the system, network design and your version of 800xA, you need to set up a new pair of connectivity servers or deploy the new connectivity to a existing pair.
OPC is simple, fast and effective regarding for operator presentation and information, alarm & event, trending etc. If it comes to the real-time demands of control and interlocking, I would prefer a more deterministic approach (controller to controller/PLC).
For data monitoring and display it is fine.
You might also need to use an OPC bridge, which increases the overhead.OPC performance is limited by its various data sources, opc bridge itself can hande around 10000 data points.
Following from this comment... You have not explained if this link is for the HMI or for transferring signals between Controllers ?
For the HMI - An OPC Connection is OK. See previous answers for how to make this work.
For transferring signals between the controllers, eg permissives, process data for control, interlocks etc. then OPC is **NOT** recommended