Question

What is the output Y in the following circuit, when all the three inputs A, B, C are first 'zero' and then 'one'?

• A. 0,0
• B. 0,1
• C. 1,0
• D. 1,1

Answer 1

Answer: (D)

1,1

Answer 2

The circuit consists of two NAND gates connected in series.

1. First NAND Gate:
   $Y_1 = \text{NAND}(A, B) = \overline{A \cdot B}$

   • For $A = 0, B = 0$:
     $Y_1 = \overline{0 \cdot 0} = \overline{0} = 1$
   • For $A = 1, B = 1$:
     $Y_1 = \overline{1 \cdot 1} = \overline{1} = 0$

2. Second NAND Gate:
   $Y = \text{NAND}(Y_1, C) = \overline{Y_1 \cdot C}$

   • When all inputs are $0:$ $C = 0$ and $Y_1 = 1$, so
     $Y = \overline{1 \cdot 0} = \overline{0} = 1$
   • When all inputs are $1:$ $C = 1$ and $Y_1 = 0$, so
     $Y = \overline{0 \cdot 1} = \overline{0} = 1$

Thus, for both cases, the output $Y$ is $1$.