GE Fanuc PLC Redundant Modbus serial slaves

I am planning to connect to a GE Fanuc 90-30 PLC using dual serial ports to my RTU for which I am writing the code. I assume the PLC is a hot backup system (HBR redundancy) with 2 CPUs and I plug a serial cable each to each CPU serial port. I plan to make my RTU the master and the PLCs slaves.

I hope someone can advise me on a few points.

- Is there a device like a splitter for 2 slaves to which I can connect one Modbus master. (How can this device know which is the active CPU?)

- Is Modbus RTU ok for this or should I need SNP?

- Or do I need 2 serial ports in my master? In which case is it advantageous to write the polling code so the program 'knows' which port connects to the active CPU and the other remains silent or let both ports poll freely but write the data to the same registers in my master (with the assumption that both slave CPU are always synchronised)

- Is it possible that there is no way to detect which is the active CPU in the slave and I always have to assume, say poll left CPU first but if no responese then poll right CPU (left is the assumed primary)

Your two PLCs will have a block of data that is continuously shared between the active and the backup PLC. There should also be a bit (or bits) somewhere in memory that indicates which PLC is in control.

If the data that your RTU master monitors is included in that block of shared data, then it doesn't really matter which PLC the RTU master monitors unless one of the PLCs is shut down. You should also include the bit(s) that indicate which PLC is in control in the data monitored by the RTU master. I would expect that (those) bit(s) to be included in the shared data block.

The easiest way to monitor both PLCs from the same serial port is to use an RS422 connection. Give each PLC a different RTU device address and in your RTU master include the ability to switch from polling one slave to the other.