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The binary output Y3 Y2 Y1 Y0 of the following logic circuit for the inputs X3 X2 X1 X0 = 1 0 1 1 is :

Y3 is X3; Y2 is the output of an XOR gate fed by X3 and X2; Y1 is the output of an XOR gate fed by Y2 and X1; Y0 is the output of an XOR gate fed by Y1 and X0

  • A.1 1 0 1
  • B.1 0 1 0
  • C.1 1 1 1
  • D.0 1 0 1

Answer: A

The output is 1 1 0 1.

The circuit is a Gray-to-binary converter: \(Y_{3} = X_{3}\), and each lower bit is the XOR of the next higher output bit with the corresponding input:

  • \(Y_{3} = X_{3} = 1\)
  • \(Y_{2} = Y_{3} \oplus X_{2} = 1 \oplus 0 = 1\)
  • \(Y_{1} = Y_{2} \oplus X_{1} = 1 \oplus 1 = 0\)
  • \(Y_{0} = Y_{1} \oplus X_{0} = 0 \oplus 1 = 1\)

Feeding each XOR with neighbouring input bits instead (binary-to-Gray) would give 1 1 1 0, which is not offered. 1 0 1 0 and 0 1 0 1 do not satisfy \(Y_{3} = X_{3} = 1\) together with the XOR chain.

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