ISCET Digital Electronics Principles Questions and Answers — Questions and Answers
Question 1: Which of the following Boolean expressions correctly represents one of De Morgan's theorems?
- A AND B
- NOT (A AND B) = (NOT A) AND (NOT B)
- NOT (A OR B) = (NOT A) OR (NOT B)
- NOT (A AND B) = (NOT A) OR (NOT B) (Correct answer)
Correct answer: NOT (A AND B) = (NOT A) OR (NOT B)
De Morgan's theorems are fundamental rules in Boolean algebra for converting between AND and OR operations with negation. One of the theorems states that the complement of a product of variables is equal to the sum of the complements of the variables. In Boolean terms, this is expressed as NOT (A AND B) = (NOT A) OR (NOT B).
Question 2: A technician is designing a circuit that needs to select one of eight data inputs and route it to a single output. Which digital logic device is most suitable for this task?
- Demultiplexer (DEMUX)
- Encoder
- Multiplexer (MUX) (Correct answer)
- Decoder
Correct answer: Multiplexer (MUX)
A Multiplexer (MUX), also known as a data selector, is a combinational logic circuit designed to switch one of several input lines through to a single common output line. An 8-to-1 multiplexer would use three select lines (2^3 = 8) to choose which of the eight inputs is passed to the output.
Question 3: In a J-K flip-flop, what is the output behavior when both J and K inputs are held HIGH (logic 1) and a clock pulse is applied?
- The output remains in its previous state.
- The output toggles to the opposite state. (Correct answer)
- The output is reset to 0 (LOW).
- The output is set to 1 (HIGH).
Correct answer: The output toggles to the opposite state.
When both J and K inputs of a J-K flip-flop are held HIGH, the flip-flop is in the 'toggle' mode. On each active clock edge, the output Q will invert to the opposite of its current state.
Question 4: A technician is using a Karnaugh map (K-map) to simplify a 4-variable Boolean expression. How many cells will the K-map contain?
- 4
- 8
- 32
- 16 (Correct answer)
Correct answer: 16
A Karnaugh map is a graphical method used to simplify Boolean expressions. The number of cells in a K-map is determined by the formula 2^n, where 'n' is the number of variables in the Boolean expression. For a 4-variable expression, the K-map will have 2^4 = 16 cells.
Question 5: What is the key difference between a latch and a flip-flop in digital sequential circuits?
- Latches are made from logic gates, while flip-flops are not.
- Latches can store more bits of data than flip-flops.
- Latches are level-triggered, while flip-flops are edge-triggered. (Correct answer)
- Latches operate asynchronously, while flip-flops operate synchronously.
Correct answer: Latches are level-triggered, while flip-flops are edge-triggered.
The fundamental difference lies in their triggering mechanism. A latch is level-triggered, meaning its output can change whenever the enable signal is active (e.g., HIGH). A flip-flop is edge-triggered, meaning its output only changes at the moment the clock signal transitions from low-to-high (rising edge) or high-to-low (falling edge).
Question 6: A technician is troubleshooting a digital logic circuit and finds that the output of a specific IC is always LOW, regardless of the inputs. The VCC and GND pins are confirmed to have the correct voltage. Which of the following is the most likely fault?
- An open circuit on one of the logic inputs.
- The output of the IC is internally shorted to ground. (Correct answer)
- The subsequent IC in the signal path has a shorted input.
- The system clock frequency is too high.
Correct answer: The output of the IC is internally shorted to ground.
If the output of an IC is permanently stuck in a LOW state (stuck-at-0 fault) even when the inputs should produce a HIGH output, and power connections are correct, it strongly indicates an internal failure. A common internal failure mode is for the output driver transistor to be shorted to the ground plane.
ISCET Digital Electronics Principles Questions and Answers
Which of the following Boolean expressions correctly represents one of De Morgan's theorems?