Logic Gates Questions and Answers

Answer: A

Correct answer: True

XOR is the inequality detector: its output is HIGH exactly when its two inputs differ.

Answer: D

Correct answer: Both (a) & (b)

XOR is its own inverse, so an equation may be rearranged freely: from \( A\oplus B=C \), XOR both sides with \(B\) to get \( A = B\oplus C \), or with \(A\) to get \( B = A\oplus C \). Both rearrangements hold, which is why the third option - \( A\oplus B\oplus C = 0 \) - is also true but is not the only one.

Answer: D

Correct answer: 4

The 74HC21 is a dual four-input AND gate: two gates of four inputs each, which fits a 14-pin package with two pins to spare for supply and ground.

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: B

Correct answer: C MOS ICs

CMOS draws current essentially only while switching, so its static power dissipation is negligible - far below TTL or ECL - and its simple structure gives a low cost per gate. That combination is why CMOS displaced the other families.

Answer: C

Correct answer: 16

The number of rows depends only on the number of inputs, not on the function: \( 2^4 = 16 \) for any four-input gate.

Answer: D

Correct answer: Three bit parity checker

The sum output of a full adder is \( A \oplus B \oplus C_{in} \) - it is 1 whenever an odd number of the three inputs is 1. That is precisely what a three-bit parity checker computes. The carry output, by contrast, is the majority function of the same three bits.

Answer: A

Correct answer: Operating speed

The operating speed of a gate is the highest frequency at which it can still follow its input - set by its propagation delay, since a gate cannot respond faster than it can switch.

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: A

Correct answer: NOR

NOR is the complement of OR by definition, so its output is the exact inverse of the OR output for every input combination. NAND inverts AND, not OR, and a NOT gate takes only one input.