u201d when the input level is greater than or equal to u2212 0 3v and the clamping transistor ntr 1 becomes off accordingly in this condition the interface circuit serves as normal input buffer on the other hand when the input level is smaller than u2212 0 3v the output cmpout of the comparator cmp 1 becomes u201c h u201d and the clamping transistor ntr 1 turns on in this condition the interface circuit serves for clamping the input level at 0v a result of simulation simulating what effect of the foregoing operation would be achieved for the ringing waveform is illustrated in fig7 in fig7 pcout corresponds to the output signal of the output buffer 1 on the side of the pc in fig1 periin corresponds to the input signal of the input buffer 5 on the side of the peripheral device cmpout corresponds to the comparison result output signal of the comparator cmp 1 and nor 10 corresponds to the output signal of the two input nor gate nor 1 in fig7 in a period a the signal periin is greater than u2212 0 3v the output cmpout of the comparator cmp 1 is u201c h u201d in a period b the signal periin is fallen to lower the level thereof below u2212 0 3v therefore after about 5.