Modbus Gateway Redundancy


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I have established communication between DCS AS Master (Yokogawa) two (ALE 111) for redundancy and our Modbus Gateway which is a data Concentrator as slave Modbus/tcp/ip through Ethernet cables. we have one Modbus Gateway connected with DCS CARD ALE 111, and the other hmi 2 connected with dcs other card ale 111.

The configuration in my system is that only one gateway can transmit data to one ALE card, and the other is on standby. When there is a case of Disconnection from GTW Server 1, the other will take over as redundancy.

My questions are:

Does the DCS request same data from both hmi 1 & 2 with separate two cards?
In principal Modbus TCP works on Client/Server architecture when you have to define the Client (Master) and Server (Slave). When Modbus TCP protocol requests data, the address identifier is IP address and Modbus Registers.

For your case, your DCS must have configuration to detect break in communication with one of the slaves so that it can switch between the two redundant communication cards. This can be done with the timeout and retries coordination in conjunction with status code that can be read from each of the Servers (Slaves).