An improved serial data protocol/format is provided in a serial multiplex system for use in an automotive vehicle electrical system which is subject to electrical noise. The protocol/format includes successive transactions between a master controller and respective remote controllers each connected to a serial multiplex bus. Each transaction is bidirectional and includes a command message transmitted by the master controller and a reply message normally transmitted by an answering remote controller. Each command message is preceded by a multibit SYNC byte. The command message includes an address byte (for a particular remote controller), a command byte, and an error check such as a cyclic redundance check byte passed on the address and command bytes. The reply message includes the address byte of the replying remote controller, a response byte indicative of its response to the command byte, and an error check such as a cyclic redundancy check byte based on the reply address and response bytes. The master controller may be caused to repeat the transmission of a command message if the address bytes, the command/response correlations or error check bytes are not verifiable, thereby enhancing the system's ability to detect and/or reject errors as might be occasioned by noise.