Question

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’s relation with 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

The question was posed to me in an interview.

My enquiry is from Fast Adder & Serial Adder topic in section Arithmetic Circuits of Digital Circuits

Answer 1

Right option is (d) The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, followed by a 40 s pulse

Best explanation: 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-versa, output is high. In this problem for the first 20uS one input is high and another is low. So, obviously output 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.