Redundant Modbus serial slaves from GE PLC

[august7]

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)

 

[djs]

First off the serial port on the 90-30 is SNP only. You will need a Modbus comms card to handle the Modbus from your RTU. This is a much better idea, place a comms card in one of the I/O racks connected to the redundant PLC processors.