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GATE 2014 EC (EC1) – Question 52

Digital Circuits · Sequential Circuits · 2 marks · Multiple choice

The logic circuit realizes the shown state diagram when Q₁ connects to XOR input A. If XOR is replaced by XNOR, which connection preserves that state diagram?

Diagram for GATE 2014 EC (EC1) question 52
  1. Connect A to $\overline Q_2$
  2. Connect A to Q₂
  3. Connect A to $\overline Q_1$ and complement S
  4. Connect A to $\overline Q_1$

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Show answer and explanation

Correct answer: (D) Connect A to $\overline Q_1$

Explanation

Originally the second flip-flop’s next-state input is Q1 XOR S. Complementing one input to an XNOR recovers the same XOR function: Q̄1 XNOR S=Q1 XOR S. Therefore connect A to **Q̄1,D** and leave S unchanged. Complementing S as well would change the function back.