Logic Gates Questions and Answers

Answer: B

The NAND expression is (A·B)'.

NAND is NOT-AND: the output is the complement of A AND B, so it is 0 only when both inputs are 1.

(A + B)' is NOR, A + B is OR, and A·B is AND. NAND and NOR are called universal gates because any logic circuit can be built from either alone.

Truth table check for NAND: inputs 00, 01 and 10 give 1, and only 11 gives 0, which is exactly the complement of AND.

Answer: C

Correct answer: carry from ( K - 1 )TH bit

A full adder at bit position k adds the two operand bits of that position together with the carry produced by position \(k-1\). It is that third input that distinguishes it from a half adder.

Answer: D

Correct answer: XOR gate

Holding one XOR input at 0 passes the other input unchanged, since \( 0\oplus X = X \); holding it at 1 inverts it, since \( 1\oplus X = X' \). That makes the XOR a controlled inverter, which no other two-input gate provides.

Answer: D

Correct answer: AND gates and OR gates

Programmable arrays implement sum-of-products expressions, so they consist of an AND plane forming the product terms and an OR plane summing them. Whether one plane, the other, or both are programmable is what distinguishes PROM, PAL and PLA.

Answer: A

Correct answer: True

In a programmable OR array each product term reaches the output through a fuse, and blowing a fuse removes that term from the function, leaving only the terms wanted.

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: the input is LOW

A NOT gate produces the complement of its input, so its output is HIGH exactly when the input is LOW. The presence of supply power is a precondition for the gate working at all, not a logical input.

Answer: B

Correct answer: OR gate immediately followed by an inverter

NOR means NOT-OR: \( (A+B)' \). Inverting the output of an OR gate produces exactly that expression.

Answer: B

Correct answer: False

Troubleshooting works from the output backwards: the output tells you whether the circuit is doing what it should, and the inputs are then checked to find why not. Watching the input alone shows nothing about the fault.

Answer: B

Correct answer: NAND

Holding one input of an AND or NAND gate LOW forces the AND function to 0, which blocks the signal on the other input. If the output is then HIGH, the gate must be inverting the blocked 0 - so it is a NAND.