PURPOSE: To detect a default between interfaces of a host device and a subordinate device, by returning data sent from the subordinate device to the host device reversely from the host device to the subordinate device to collate them with each other.

CONSTITUTION: The data of the subordinate device which are inputted from a BUS IN by a strobe signal S1 are stored in registers 1W4 successively through a counter 16. When receiving the data, an IOC (I/O controller) host device sends the data to a BUS OUT and responds by a strobe signal S2. The signal S2 is inputted to an FF 11 and a counter 14, and a decoder 13 sends signals to open gates 5W8 successively. The returned data from the IOC are inputted to a comparator 9 and compared with the data stored in a register of which gate is opened by the specification from the decoder 13. The output of an OR circuit 10 is changed to "1" and "0". If the data of the BUS OUT are incorrect and the compared output is kept dissident the FF 11 is set up by the signal S2 and an AND circuit 12 outputs "1" to inform the data error.