Digital (Un)Logic

A recent discussion with a relative of mine about American students in general reminded me of this incident that happened in summer, when I was teaching Digital Logic to second year students.

(Those of you not familiar with the concept of an electrical signal might not get this!)

The topic of the day was propogation delay of gates. The lecture had just finished and I couldn't wait to get away from the class and go to watch the world cup game between USA and Czech Republic, when this guy comes up with a doubt. Those who know me will know this was the worst time to catch me, football (soccer) games are after all no ordinary matters! I tried to be quick about the whole thing.

He had some trouble understanding how the signal propogated through a particular circuit, where the input was connected to a NOT gate. The input switched from zero to one. I thought the analysis was fairly straight-forward, but no, he didn't get it.

"This input," he began, "it changes from zero to one, right?"

"Yes." I replied.

"So, the output of the NOT gate goes from one to zero?"

"Yes!"

"So why doesn't this output go back to one??"

It certainly would be interesting if NOT gates began to do such things to the output, but on balance, not such a good thing because the job of a TA would then become infinitely more difficult!

"Theres no reason why it should go back to one!"

"I see. How about the input?"

I blinked. Atleast this one was a straight-forward question. "Well, what about the input? It changes from zero to one."

"Yes, it does, but shouldn't it go back to zero?"

I really had no idea what he was upto by this point. Maybe he just wanted to irritate me with such inane questions, something that others had experienced at different points in their teaching assignments. Maybe he wanted to talk about the inherent randomness of the universe. Or maybe...

"If the external circuit driving this one has its conditions changed, then maybe that can get reflected as a switch back to zero, but otherwise, the input will not change to zero all on its own."

The look of bemusement on his face suggested he really did not get it. By this time, he had started using his hands as an aid to speech, treating the signal as if it were some emotion gushing forth from the heart.

"The thing I do not get is," he continued, with his hands moving away from his chest in the manner they sometimes do when one is professing one's love to a lover, "after the input has finished giving the 'one' to the NOT gate, shouldn't it get extinguished and go back to zero?"

Four and a half vodkas (neat) wouldn't have done half as much damage to me!