A data communications system having a plurality of processing devices connected to a serial via a serial bus interface. The processing device provides a communication control line which connects each processing device in order and performs the transfer of transmission right of the serial bus, a transmission right management unit which is connected to the communication control line, performs the transfer of the transmission right and manages the state of the transmission right within its own processing device, a data transmission unit which starts data transmission when the transmission right is valid and holds the data transmission when the transmission right is invalid, and a state initialization unit which sets the initial state of the transmission right so that the transmission right of only one processing device is made to be valid at the initialization.