Logic Gates Questions and Answers

Answer: D

Correct answer: both (a) and (c)

With inputs 1 and 0, OR gives \(1+0=1\) and NAND gives \((1\cdot 0)'=0'=1\). AND gives \(1\cdot 0=0\), so it is excluded.

Answer: C

Correct answer: Fan-in of a gate is always equal to fan-out of the same gate

Fan-in is the number of inputs a gate has and fan-out the number of gate inputs its output can drive; the two are independent properties and there is no reason for them to be equal. The other three statements are the standard definitions of propagation delay, noise immunity and operating speed.

Answer: D

Correct answer: X NOR 1

Work each one out. \( X\,\text{NAND}\,X=(X\cdot X)'=X' \); \( X\,\text{NOR}\,X=(X+X)'=X' \); \( X\,\text{NAND}\,1=(X\cdot 1)'=X' \). But \( X\,\text{NOR}\,1=(X+1)'=1'=0 \), which is a constant, not \(X'\).

Answer: A

Correct answer: NOT

By De Morgan's law \( (AB)' = A' + B' \), so a NAND is an OR gate whose inputs have been inverted. Putting a NOT gate on each input of an OR therefore gives NAND.

Answer: D

Correct answer: XOR

XNOR is by definition the complement of XOR, so an XOR followed by an inverter produces it. It is 1 when its inputs agree, which is why it also serves as an equality detector.

Answer: A

Correct answer: 1 bit

A one-to-four demultiplexer sends a single data bit to one of four outputs, so the memory needs four locations - addressed by the two select lines - each holding just the one data bit.

Answer: A

Correct answer: NAND

NAND means NOT-AND, so inverting the output of an AND gate is exactly how it is formed.

Answer: A

Correct answer: AND gate

An AND gate outputs 1 only when every input is 1 at the same moment, so it signals the coincidence of all its inputs. That is why it is used as a coincidence detector in counting and timing circuits.

Answer: B

Correct answer: Truth table

A truth table lists the logical output for every combination of logical inputs. A function table does the same job in terms of electrical levels - HIGH and LOW - which is the distinction the next question turns on.

Answer: D

Correct answer: OR gate

In negative logic the roles of the two voltage levels are exchanged, and by De Morgan's law that turns AND into OR: \( (A'\cdot B')' = A + B \). So a gate that is AND for positive logic is OR for negative logic.