GEFanuc 90-70 with (3) gbc's, redundnacy?

I am hoping someone can help me out. I have 2 sets of 90-70 plc's each containing (1) ethernet controller , (3) Genius bus controllers and ofcourse the power supplies. The first GBC on both racks I plan to use for synchronization and communication between the two cpu's. The second GBC has 18 genius blocks on its bus. The third GBC has 11 genius blocks on its bus. The application is used for a 4-leg bop lift with encoders on each corner (4) and will be using only 2 encoders for the logic to set up, down and slow down limits. The main purpose for the redundant plc is ofcourse to keep up with all inputs the master plc is seeing in case of a fault in any of the GBC's or the master plc. I know we can, with the HHM, program each individual genius block to look for the master GBC (31) status bit and if the bit is low then the genius block will send and recieve data from the redundant GBC (30). But I see a problem with a GBC(31) on the master going bad and its genius blocks then look to the second GBC(30) to operate. Then the system would be split. I have never applied any redundancy before with plc's and could use some help. Thanks

You would have to monitor the ready bits for drop 30 and drop 31(on both genius busses), if you don't have two drop 31s then you would have to give control to the backup CPU and set an alarm for getting the failed equipment back in business. The same goes for either drop 30, you would have to disable redundancy and set an alarm. Obviously you can't enable bus 1 to the master and bus 2 to the ghost.

On the Comm GBC between the CPUs, are you using Comm Reqs or Global data?