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GATE 2021 CS (CS2) – Question 38

Digital Logic · Sequential Circuits · 2 marks · Multiple choice

A synchronous circuit replaces the first 1 of every maximal run of consecutive ones by 0, leaving other bits unchanged. It is a two-state Mealy machine, initially state s=0. Let b be current input, t next state, and y output. Which equations implement it?

  1. t=s+b, y=sb
  2. t=b, y=sb
  3. t=b, y=s′b′
  4. t=s+b, y=s′b′

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Correct answer: (B) t=b, y=sb

Explanation

Let s remember the preceding input bit, so t=b. The first one in a run has b=1,s=0 and must output zero; subsequent ones have b=s=1 and output one. Every zero input outputs zero. Thus y=s AND b, giving B.

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