PURPOSE: To easily perform test and diagnosis for a package-mounted bidirectional driver/receiver, by obtaining an input logical value of the driver independently of the state on a bidirectional bus.

CONSTITUTION: When a signal line 151 indicating inversion goes to "1", the transmission logical value of a driver 110 appears at a receiver 120 of itself independently of the transmission logical value of the present opposing driver 210. When the signal line 151 indicating the inversion is at "0", the inverting function of an inversion selection gate 150 is made ineffective and the input logical value of the driver 110 is inputted to a threshold level converter 130 as it is, then the operation of a bidirectional driver/receiver 100 is the same as that of conventional systems. Thus, the executing the test and diagnosis of a data processor using this bidirectional driver/receiver, the inversion indication signal is made to "1" to operate one of package-mounted driver/receivers, allowing to improve the resolution at failure point-out of the test diagnostic program.