Electronics and Communication Engineering - Exam Questions Papers - Discussion

3. 

The circuit shown in the figure has 4 boxes each described by inputs P, Q, R and outputs Y, Z with
Y = P ⊕ Q ⊕ R, Z = RQ + P R + Q P

The circuit is a

[A]. 4 bit adder giving P + Q
[B]. 4 bit subtractor giving P - Q
[C]. 4 bit subtractor giving Q - P
[D]. 4 bit adder giving P + Q + R

Answer: Option B

Explanation:

Let P = 1101 Q = 1101

Yn = Pn ⊕ Qn ⊕ Rn

Z = Rn Qn + Pn Rn + Qn Pn

Constructing truth table

So that, Rn + 1 = Zn 1 ≥ n ≥ 3

Z4 = R5(MSB)

Hence, output is 00010 which show that it is a 4 bit subtractor giving P - Q.


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