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Modbus Redundancy: Stand-By Server/Slave
The correct working of the stand-by slave/server for double line modbus (redundancy)

Dear all,

please could you clarify to me what should be the correct working of the stand-by slave/server for double line modbus (redundancy)?

Practically my question is: is it existing a guideline or standard to be applied when we talk of modbus redundancy?

Our customer require that when the master DCS use Master line A, our side must provide on the line B the same value for every AO sent on line A (AO are Holding registers sent from DCS using line A to our PLC).

Note: Redundant modbus by Siemens automatically provide the best by allowing also cross-communication and allow the remote DCS to read/write from each line to each PLC CPU. Rockwell (trought Prosoft card) allow cross-communication but for Holding registers wrote by line A are not automatically syncronized into the line B card.

I simply want to know if when talking of Modbus Redundancy (that I discovered is not a concept applicable on Modbus), the server in stand-by must be syncronized to the other master server inside PLC? Or if the syncronization has to be achieved by the master DCS that must write Holding registers to both servers?

Thanks in advance to can clarify.