PURPOSE: To secure the early detection for the fault of the printing head by delivering the output of the printing head driver to the two-way data bus and then comparing the output signal with the printer data.

CONSTITUTION: Latch circuit 2 holds the printer data delivered onto data bus 1. The output signal sent from circuit 2 is supplied to head driver 5, and printing head 4 is driven by the pulse of a fixed time duration produced at printing width setting circuit 9. Output 6 of driver 5 is supplied also to level converter circuit 7 to receive the level conversion. The output signal of circuit 7 receives the control by tristate gate 11 which opens/closes the gate via the signal same as the output signal given from circuit 9. Thus the printer data and the output signal from driver 5 are delivered alternately and in pile onto bus 1. The constant comparison is given between these two signals, thus detecting the fault of head 4 in an easy way.