Can Modbus support redundant masters?

The Modbus protocol does not support redundant masters, however ProSoft has had customers setup a redundant Modbus network using the inRAX MVI46-MCM, MVI56-MCM, MVI71-MCM, 3100-MCM, and 3150-MCM modules. Only ProSoft products that support configuration through ladder logic can be used for a redundant application. The basis behind this configuration is you have one port communicating to various slaves on a Modbus network. During this communication, you will want to monitor for command errors.

In your PLC program, when you have determined (specified by you the programmer) that you have lost communications on the Modbus network, you would then disable the port you are currently communicating on, then enable the port you wish to communicate on.

For even more redundancy, this can also be done using 2 ProSoft modules. The same theory is involved. You will want to disable the communications on one module when errors have occurred, then enable the communications with the other module. In this configuration keep in mind that the data that is being transferred from the module to the processor (your read data), will reside in 2 different databases.

You will also need to have a way of knowing what card is currently the master, and what data value to look at based on what card is the master. Since two cards are used, you will need to have 2 independent read data arrays (from module to processor), and because of this you will have 2 variables in your processor that will represent the same point. 

This programming must be done by the user, and is not supported by ProSoft. The timing, and error conditions are all specified by the user for this type of an application. ..

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