Logic Gates Questions and Answers

Answer: C

Correct answer: Both (a) & (b)

A demultiplexer routes one input to a selected output, and if the select lines are stepped through in turn while bits arrive one after another, the same circuit distributes a serial stream across parallel outputs. Selecting one of many inputs is the multiplexer, which is the reverse operation.

Answer: B

Correct answer: Switches connected in parallel

With switches in parallel, closing any one of them completes the circuit, which is the OR condition. Switches in series need every one closed, which gives AND.

Answer: A

Correct answer: OR gates and AND gates only

A set of gates is functionally complete only if it can produce complementation. AND and OR alone can never invert a signal, so functions involving a complement cannot be built. Each of the other three sets can invert: NOT is given outright, XOR inverts when one input is held at 1, and NAND is universal by itself.

Answer: B

Correct answer: m-1

In an m-bit ripple carry adder the lowest bit has no incoming carry and needs only a half adder; the other \( m-1 \) bits each add two bits and a carry, which is the full adder's job.

Answer: A

Correct answer: combinational logic circuits

A parallel adder's outputs depend only on the bits currently present at its inputs - it holds no state and needs no clock, which is the definition of a combinational circuit. Sequential circuits are those whose output depends also on what has gone before.

Answer: C

Correct answer: AND-OR

A multiplexer forms the AND of each data input with its decoded select condition, and then ORs the results together, so exactly one input reaches the output. That two-level AND-OR structure is the standard implementation.

Answer: C

Correct answer: High if all of its inputs are low

NOR is the complement of OR, and OR is 0 only when every input is 0. So NOR is 1 in exactly that one case - all inputs low - and 0 otherwise.

Answer: D

Correct answer: 16

A truth table has one row for each combination of input values, and n inputs give \( 2^n \) combinations. With four inputs that is \( 2^4 = 16 \).

Answer: B

Correct answer: AND gate

Toggling means inverting, and inversion requires a gate that can produce a complement. NAND, NOR and XOR can each be wired to invert - tie the inputs together, or hold one XOR input at 1 - but an AND gate with its inputs tied together simply reproduces the input, so it can never toggle.

Answer: A

Correct answer: Function table

A function table records the electrical output - HIGH or LOW - for each combination of electrical inputs, without committing to which level means 1. A truth table is the logical version of the same table, obtained once that assignment is made.