gateways must be used to isolate the redundant buses from each other these gateways in turn are connected eventually to a common bus master the two redundant bus gateways are on the same level in the network hierarchy and therefore unaware that they poll transponders in common packets arriving at these gateways bound for their bus slaves are passed on by both bus masters since these gateways have essentially created a duplicate or redundant packet the destination device will receive the packet regardless of a single bus failure one of the packets will be discarded as a duplicate when both buses are functional transponder produced packets will only need to be sent on one of the redundant buses the fiber optic bus provides redundancy of communications paths but does not allow both paths to be used simultaneously this means the bus master must coordinate bus polling packets arriving at the gateway can be sent over one of the fiber paths at the discretion of the bus master duplicate packets are not created here as they were in the electrical configuration above the configuration of dual fiber optic bus masters occurs when redundant electrical buses are converted to fiber optic medium or when two independent system masters drive each side of the fiber optic bus in either case the gateways pass packets addresses for their slave devices since each gateway can only drive one of the buses the packet is not further duplicated special considerations apply when independent system masters.