PURPOSE: To save man-power, to efficiently discriminate propriety, and to clearly discriminate even a defective part, by feeding back an output signal of a comparison circuit to a central processing unit via an interface for the discrimination of the propriety of the interface.

CONSTITUTION: When data signals of BITO 7 are sequentially applied from a printer interface 1 to a latch circuit 2, a control signal STB is applied to the latch circuit 2 and a counter decoder CD3. When the output signal of the latch circuit 2 and that of the CD3 are compared at a comparison circuit 4, the output signal is indicated by a signal having a waveform as shown by C. The signal C is applied to a central processing unit via the interface 1 from an OR circuit 5 as a discrimination signal. The performance of normal operation is displayed at, e.g., a display device and the computer itself discriminates the propriety of the interface 1.