Abstract: A data communication system for use on a motor vehicle includes at least one master unit and at least one slave unit which are connected to a communication bus. When the master unit is enabled, the master controller of the master unit outputs connection request demand data to the slave unit. The slave unit has a slave controller which transmits its own connection request information to the master unit in response to the connection request demand data from the master unit. The slave controller transmits, to the master unit, first change information indicating whether the connection information stored in a first memory and connection information of the slave unit that has been transmitted to the master unit are identical to each other or not. The master controller transmits some or all of connection information stored in a second memory to slave units based on the first change information of the slave units.