ISCET Digital Electronics Principles 5 — Questions and Answers
Question 1: Which type of logic circuit has outputs that depend only on present inputs, with no memory?
- Sequential circuit
- Combinational circuit (Correct answer)
- Synchronous circuit
- Finite state machine
Correct answer: Combinational circuit
Combinational circuits have no feedback or storage; outputs are determined solely by the current input combination.
Question 2: What is the purpose of a priority encoder?
- Converts binary to BCD
- Outputs the binary code of the highest-priority active input (Correct answer)
- Selects one of many data sources
- Detects parity errors
Correct answer: Outputs the binary code of the highest-priority active input
A priority encoder resolves simultaneous inputs by outputting the binary address of the highest-priority (typically highest-numbered) active input.
Question 3: In synchronous sequential circuits, what ensures all flip-flops change state at the same time?
- Feedback loops
- A common clock signal (Correct answer)
- Asynchronous reset
- Propagation delay matching
Correct answer: A common clock signal
A shared clock signal drives all flip-flops simultaneously, making the circuit synchronous and predictable.
Question 4: Which BCD encoding assigns weights 8-4-2-1 to the four bit positions?
- Excess-3
- Gray code
- Natural BCD (8421) (Correct answer)
- 2-out-of-5 code
Correct answer: Natural BCD (8421)
Natural BCD (8421) assigns positional weights 8, 4, 2, 1 from MSB to LSB, directly representing decimal digits 0–9.
Question 5: What causes a race condition (hazard) in combinational logic?
- Insufficient power supply
- Unequal propagation delays causing momentary incorrect output glitches (Correct answer)
- Using too many logic gates
- Clock skew in flip-flops
Correct answer: Unequal propagation delays causing momentary incorrect output glitches
A logic hazard occurs when signal paths through a circuit have unequal delays, producing brief spurious output transitions.
Question 6: A JK flip-flop with J=1 and K=1 on the active clock edge will:
- Set Q to 1
- Reset Q to 0
- Toggle Q to its complement (Correct answer)
- Hold Q unchanged
Correct answer: Toggle Q to its complement
J=1, K=1 is the toggle condition for a JK flip-flop; Q switches to its complement on each active clock edge.
Question 7: Which digital-to-analog converter (DAC) architecture uses a ladder network of only two resistor values (R and 2R)?
- Binary-weighted resistor DAC
- R-2R ladder DAC (Correct answer)
- Delta-sigma DAC
- Flash DAC
Correct answer: R-2R ladder DAC
The R-2R ladder DAC requires only two resistor values and scales well to high bit counts, unlike the binary-weighted design.
ISCET Digital Electronics Principles 5
Which type of logic circuit has outputs that depend only on present inputs, with no memory?