The system includes optical bus-bridging devices for observing the modes
of said electric buses and the modes of said optical fibers while said
electric buses have not been driven (OFF mode), so that the modes of the
two electric buses connected through optical fibers are brought into
agreement and that the buses can be simultaneously driven by a plurality
of nodes. While one or both of said electric buses have been driven (ON
mode) by the nodes connected thereto, an optical output has been
continuously produced from the buses that are being driven to said
optical fibers, and while light has been inputted from said optical
fibers, the modes of said buses are not observed, but an electric output
is produced to the electric bus of the side to which light is inputted to
drive the bus. The optical bus-bridging device changes the mode of the
electric bus when the optical fiber does not change within a
predetermined period of time after the optical bus-bridging device has
outputted a signal to the optical fiber.