Data transmission system and method

In a data transmission device 1, before starting of data transmission, a data conduct device 6 detects an external bus 5, and an address comparator section 8 compares the value of the address signal line 5a with the output values of the parameter memory section 7. As the result of the comparison, when it is judged that the data to be transmitted is present on the external bus 5, the address comparator section 8 transmits a write-execution signal to a bus control unit 11 so that the bus control unit 11 writes the data present on the external bus 5 together with the valid bid thereof into the data memory device 10. In the period of data transmission, when the data assigned to the address produced by an address generator 9 is present in the data memory device 10, the data is transmitted to the external destination 2, and on the other hand, when the data assigned to the address is absent in the data memory device 10, the bus control unit 11 gains access to the external memory 4 for obtaining the data corresponding to the address, so that the data obtained from the external memory 4 is transmitted to the external destination 2.