BEE Digital Circuits 4 — Questions and Answers
Question 1: Which of the following is a universal gate because it can implement any Boolean function on its own?
- AND
- OR
- XNOR
- NOR (Correct answer)
Correct answer: NOR
NOR is a universal gate; using only NOR gates, you can construct NOT, AND, OR, and any other logic function.
Question 2: In an SR latch built from NOR gates, what happens when both S=1 and R=1 are applied simultaneously?
- The latch holds its previous state
- Q=1 and Q̄=1 (forbidden state)
- Q=0 and Q̄=0 (forbidden state) (Correct answer)
- The latch oscillates
Correct answer: Q=0 and Q̄=0 (forbidden state)
With S=R=1 in a NOR-based SR latch, both outputs are forced to 0, which is the forbidden/invalid state because Q and Q̄ are no longer complementary.
Question 3: A binary ripple counter uses 4 T flip-flops. After 19 clock pulses, what is the counter output (MSB to LSB)?
- 0011 (Correct answer)
- 1001
- 0100
- 1010
Correct answer: 0011
19 in 4-bit binary is 10011₂; a 4-bit counter truncates to the lower 4 bits: 0011 (19 mod 16 = 3 = 0011₂).
Question 4: What does the term 'fan-out' refer to in digital logic?
- The number of inputs a gate can accept
- The maximum number of gate inputs that a gate output can reliably drive (Correct answer)
- The speed at which signals propagate
- The power dissipation per output
Correct answer: The maximum number of gate inputs that a gate output can reliably drive
Fan-out specifies how many standard gate inputs the output of a given gate can drive while maintaining valid logic levels.
Question 5: Which circuit converts a BCD (Binary Coded Decimal) input to a seven-segment display output?
- Encoder
- Decoder (Correct answer)
- Multiplexer
- Comparator
Correct answer: Decoder
A BCD-to-seven-segment decoder takes a 4-bit BCD input and produces the 7 output signals needed to drive the correct segments of a seven-segment display.
Question 6: In a Moore machine, the output depends on:
- Current inputs only
- Current state and current inputs
- Current state only (Correct answer)
- Previous state and previous inputs
Correct answer: Current state only
In a Moore machine, outputs are functions of the current state only, not the current inputs.
Question 7: A 4-bit magnitude comparator has inputs A=1010 and B=1010. Which output is asserted?
- A > B
- A < B
- A = B (Correct answer)
- Both A>B and A<B
Correct answer: A = B
Since A and B are identical (both equal 10 in decimal), the A=B output of the comparator is asserted.
BEE Digital Circuits 4
Which of the following is a universal gate because it can implement any Boolean function on its own?