PURPOSE: To minimize the effect on the data transmission by a failed communication bus by selecting a bus in which a preamble at the head of each message is agreed with the reference preamble out of two buses to transmit the same message.

CONSTITUTION: A first preamble 1 and a second preamble 2 from buses B1, B2 are compared with the reference preamble by a comparator 48 in a MUX (multiplexer) 46. If the preambles 1, 2 are agreed with the reference preamble, the MUX 46 does not switch the bus which is presently selected to give the message. When neither of the preambles 1, 2 is agreed with the reference preamble, a transmission error is present. When the preamble from the presently selected bus is not agreed in a switch 50 to the output from the MUX 46, but the other preamble is agreed, the other bus is selected to give a message to a parallel/series converter of a receiver.