PURPOSE: To simplify the circuit constitution and to save man-power, by feeding back an AND output of the 2nd AND circuit to a central processing device via an interface to discriminate the propriety of the interface.

CONSTITUTION: When a data signal in, e.g., 8-bit is applied to a latch circuit, if the level of an output signal of an AND circuit 3 and an OR circuit 4 goes both to 1, an outut level of an FF circuit 5 goes also to 1 and the level of an output signal of an AND circuit 6 goes to 1. This output signal is applied to a CPU via a printer interface 1 as a discriminating signal for the propriety of this check device. While the output signal of the AND circuit 6 remains level 1, the performance of normal operation is displayed with a display device, for example, and a computer itself discriminates the propriety of the interface.