Abstract: A data bus system includes a plurality of serially connected active terminals configured to receive data and either retransmit the received data or transmit new data to the next adjacent terminal. The terminals are configured in a diagnostic mode to configure the system optimally at turn-on or in the event system faults occur, and are configured in a user access mode once optimum configuration is attained. The system is under control by one terminal at any one time during the diagnostic mode and any terminal is capable of being the control terminal. For operation in the user access mode a cyclic period is defined during which a portion of the period is set aside for synchronous transmissions and another portion of the period is set aside for asynchronous transmissions. Both synchronous and asynchronous users are thereby accommodated by the bus.