BEE Digital Logic and Systems 4 — Questions and Answers
Question 1: Which sequential circuit element is used to detect a specific binary sequence in a serial data stream?
- Multiplexer
- Shift register
- Sequence detector (finite state machine) (Correct answer)
- Priority encoder
Correct answer: Sequence detector (finite state machine)
A finite state machine (sequence detector) is designed to recognize a specific pattern in a serial bit stream by transitioning through defined states.
Question 2: How many flip-flops are required to build a MOD-16 counter?
- 2
- 3
- 4 (Correct answer)
- 8
Correct answer: 4
A MOD-16 counter cycles through 16 states (0–15), requiring 4 flip-flops since 2⁴ = 16.
Question 3: Gray code is preferred over standard binary in mechanical encoders because:
- It uses fewer bits
- Only one bit changes between consecutive values (Correct answer)
- It supports fractional numbers
- It is easier to add
Correct answer: Only one bit changes between consecutive values
Gray code changes only one bit between consecutive values, preventing erroneous intermediate readings when multiple bits would change simultaneously in binary.
Question 4: What Boolean expression represents an Exclusive-OR (XOR) gate with inputs A and B?
- AB + A'B'
- A'B + AB' (Correct answer)
- AB + A'B + AB'
- A + B
Correct answer: A'B + AB'
XOR outputs 1 when the inputs differ: A'B (A=0,B=1) + AB' (A=1,B=0), written as A ⊕ B.
Question 5: In a 4-to-1 multiplexer, how many select lines are needed?
- 1
- 2 (Correct answer)
- 3
- 4
Correct answer: 2
A 4-to-1 MUX has 4 data inputs requiring 2 select lines since 2² = 4 possible selections.
Question 6: Which logic family is known for the highest noise immunity among TTL and CMOS?
- TTL (74xx series)
- ECL (Emitter-Coupled Logic)
- CMOS (Correct answer)
- RTL (Resistor-Transistor Logic)
Correct answer: CMOS
CMOS logic has high noise immunity because its switching threshold is approximately at VDD/2, and its output swings rail-to-rail.
Question 7: A Johnson counter with 4 flip-flops has how many unique states in its sequence?
- 4
- 8 (Correct answer)
- 16
- 12
Correct answer: 8
A Johnson (twisted ring) counter with n flip-flops produces 2n unique states, so 4 flip-flops give 8 states.
BEE Digital Logic and Systems 4
Which sequential circuit element is used to detect a specific binary sequence in a serial data stream?