Logic Gates Questions and Answers

Answer: C

Correct answer: 4

Write \( F=(x'+y')(z+w) \) and let \( A = x\,\text{NAND}\,y = x'+y' \). Then

\[ F = A(z+w) = Az + Aw = \big((Az)'\cdot(Aw)'\big)' \]

so \( (Az)' \) and \( (Aw)' \) are two more NAND gates and a fourth NAND combines them. Four gates in all: \( \text{NAND}(x,y) \), \( \text{NAND}(A,z) \), \( \text{NAND}(A,w) \) and \( \text{NAND} \) of the last two.

Answer: B

Correct answer: Two

A function table records electrical levels only. Assigning HIGH to 1 and LOW to 0 gives the positive-logic truth table; assigning them the other way round gives the negative-logic one. So each function table yields exactly two truth tables.

Answer: A

Correct answer: is slower

A serial adder handles one bit pair per clock, so an n-bit addition takes n clock periods. A parallel adder presents all bits at once and completes in a single propagation delay. The serial adder is simpler and smaller, which is the trade it makes.

Answer: B

Correct answer: 4

The control inputs select which data input is passed through, so k control lines address \( 2^k \) data inputs. With two control inputs that is \( 2^2 = 4 \).

Answer: C

Correct answer: 16

Each input can be 0 or 1 independently, so n inputs give \( 2^n \) combinations, and \( 2^4 = 16 \).

Answer: A

Correct answer: input combination at the time

A combinational circuit has no memory: its output at any moment is a function of the inputs present at that moment alone. Any dependence on previous inputs or outputs makes the circuit sequential.

Answer: B

Correct answer: to select 1 out of N input data sources and to transmit it to a single channel

A multiplexer is a data selector: the control lines choose one of its inputs and connect it to the single output. The reverse operation - one input to one of many outputs - is the demultiplexer.

Answer: A

Correct answer: Encoder

An encoder takes an active input line and produces the binary code that identifies it, so it is the circuit that generates a binary word. A decoder performs the inverse operation: it takes the binary word and activates the one output line it names.

Answer: A

Correct answer: Multiplexer

Stepping a multiplexer's select lines through their values in turn presents its parallel inputs one after another at the single output, which is parallel to serial conversion. A demultiplexer performs the reverse.

Answer: C

Correct answer: Parallel adder

In a ripple carry chain each stage must wait for the carry from the one below it, so the delay accumulates. The question's intended contrast is with a parallel adder, in which the bits are presented and added simultaneously rather than passed along a serial chain.