PURPOSE: To equalize the priority of service request to channels, by switching the fetched address of a selection signal to a tag line alternately and dynamically in response to the operation of a controller.

CONSTITUTION: When a selection signal SELECT IN is inputted from a tag line T1, it is inputted to an AND circuit 13 through an AND circuit 10 and an OR circuit 12. Thus, a device signal inputted to a receiver R is applied to an internal selection circuit 5 and if they coincide, an AND circuit 15 is reset to fetch this selection signal. If not coincided, the AND circuit 15 transmits this selection signal to an OR circuit 16 to apply this selection signal to the next controller. In this case, a driver D outputs the device number signal to apply it to the next controller. When a command chain broken signal is applied to a termination control section 6, the selection control is made with the selection signal SELECT OUT applied via a tag line T2.