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Combinational Logic Analysis - Technical MCQs

Q1:

One positive pulse with tw = 75 µs is applied to one of the inputs of an exclusive-OR circuit. A second positive pulse with tw = 15 µs is applied to the other input beginning 20 µs after the leading edge of the first pulse. Which statement describes the output in relation to the inputs?

A The exclusive-OR output is a 20 s pulse followed by a 40 s pulse, with a separation of 15 s between the pulses.

B The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, with a separation of 40 s between the pulses.

C The exclusive-OR output is a 15 s pulse followed by a 40 s pulse.

D *The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, followed by a 40 s pulse.

ANS:D - *The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, followed by a 40 s pulse.

When both the input pulses are high or low X-OR output is low.but when one of the input is high and another is low or vice-verse output is high. In this problem for the first 20uS one input is high and another is low, so obviously out put is a high. for next 15uS both the input is high so output is low and for remaining 40uS(75-20-15) first input is still high and second one is low so output is high.