1. The basic building block of digital circuits is
A) Diode
B) Transistor
C) Logic gate
D) Resistor
Answer: C
Explanation: All digital systems are built using logic gates like AND, OR, NOT, etc.
2. A logic gate that outputs 1 only when all inputs are 1 is
A) OR gate
B) AND gate
C) NOR gate
D) XOR gate
Answer: B
Explanation: The AND gate gives a high output only if all its inputs are high.
3. The output of an OR gate is 1 when
A) All inputs are 0
B) Any one input is 1
C) All inputs are 1
D) Inputs are equal
Answer: B
Explanation: OR gate gives output 1 if at least one input is 1.
4. A NOT gate is also known as
A) Buffer
B) Inverter
C) Complementer
D) Decoder
Answer: B
Explanation: NOT gate inverts the input; hence it’s called an inverter.
5. Boolean algebra is used to
A) Design analog circuits
B) Simplify logic expressions
C) Convert AC to DC
D) Build amplifiers
Answer: B
Explanation: Boolean algebra simplifies logical operations in digital circuits.
6. The binary equivalent of decimal 13 is
A) 1101
B) 1010
C) 1110
D) 1001
Answer: A
Explanation: 13 = (8+4+1) = 1101₂.
7. The decimal equivalent of binary 1010 is
A) 8
B) 9
C) 10
D) 12
Answer: C
Explanation: 1010₂ = (8 + 2) = 10₁₀.
8. The 2’s complement of 1010 is
A) 0110
B) 1010
C) 0101
D) 0110
Answer: D
Explanation: 1’s complement of 1010 = 0101; add 1 → 0110.
9. In a half adder, the two outputs are
A) Sum and Carry
B) Product and Sum
C) Difference and Borrow
D) AND and OR
Answer: A
Explanation: Half adder produces two outputs — sum and carry.
10. A full adder has how many inputs?
A) 2
B) 3
C) 4
D) 1
Answer: B
Explanation: Full adder adds three bits (A, B, and Cin).
11. The Boolean expression for XOR gate is
A) A + B
B) A·B
C) A’B + AB’
D) (A + B)’
Answer: C
Explanation: XOR = A’B + AB’ gives high output only when inputs differ.
12. NAND gate is a combination of
A) AND + NOT
B) OR + NOT
C) NOR + NOT
D) XOR + NOT
Answer: A
Explanation: NAND = NOT(AND).
13. NOR gate is a combination of
A) OR + NOT
B) AND + NOT
C) XOR + NOT
D) NAND + NOT
Answer: A
Explanation: NOR = NOT(OR).
14. Universal gates are
A) AND, OR
B) NAND, NOR
C) XOR, XNOR
D) NOT, AND
Answer: B
Explanation: NAND and NOR can implement any logic function.
15. The logic circuit that detects equality of two bits is
A) AND
B) OR
C) XNOR
D) XOR
Answer: C
Explanation: XNOR gives 1 when both inputs are same.
16. The smallest unit of digital information is
A) Byte
B) Nibble
C) Bit
D) Word
Answer: C
Explanation: Bit (binary digit) is the smallest unit — 0 or 1.
17. 1 Byte equals
A) 4 bits
B) 8 bits
C) 16 bits
D) 10 bits
Answer: B
Explanation: 1 byte = 8 bits.
18. The hexadecimal number system base is
A) 8
B) 10
C) 12
D) 16
Answer: D
Explanation: Hexadecimal uses 16 symbols (0–9, A–F).
19. The octal equivalent of binary 110101 is
A) 65
B) 55
C) 65
D) 64
Answer: A
Explanation: Grouping: 110 101 → 6 5 = 65₈.
20. Flip-flops are used to
A) Amplify signals
B) Store binary information
C) Perform arithmetic
D) Convert AC to DC
Answer: B
Explanation: Flip-flops store 1-bit of binary information.
21. A JK flip-flop toggles when
A) J = K = 0
B) J = 1, K = 0
C) J = K = 1
D) J = 0, K = 1
Answer: C
Explanation: When both inputs are 1, JK flip-flop toggles output.
22. Which flip-flop has only one input?
A) SR
B) JK
C) T
D) D
Answer: D
Explanation: D flip-flop has only one input ‘D’ (Data or Delay).
23. In digital systems, 0 and 1 are represented by
A) Voltage levels
B) Frequencies
C) Current levels
D) Resistance
Answer: A
Explanation: Logic 0 and 1 correspond to low and high voltage levels.
24. A multiplexer has
A) One input, many outputs
B) Many inputs, one output
C) One input, one output
D) Many inputs, many outputs
Answer: B
Explanation: Multiplexer selects one of many inputs to a single output.
25. A demultiplexer has
A) One input, many outputs
B) Many inputs, one output
C) One input, one output
D) Many inputs, many outputs
Answer: A
Explanation: Demultiplexer routes one input to one of many outputs.
26. A 3-to-8 decoder has how many output lines?
A) 3
B) 8
C) 6
D) 4
Answer: B
Explanation: Decoder produces 2ⁿ outputs for n inputs → 2³ = 8 outputs.
27. The number of select lines in 8-to-1 multiplexer is
A) 2
B) 3
C) 4
D) 5
Answer: B
Explanation: 2ⁿ = 8 ⇒ n = 3 select lines.
28. Which device converts binary to decimal?
A) Encoder
B) Decoder
C) Counter
D) Comparator
Answer: B
Explanation: Decoder converts binary input into corresponding decimal output.
29. A ring counter is a type of
A) Shift register
B) Decoder
C) Multiplexer
D) Adder
Answer: A
Explanation: Ring counter is a shift register with feedback connection.
30. The 4-bit binary counter counts up to
A) 8
B) 15
C) 16
D) 10
Answer: C
Explanation: 4 bits can represent 2⁴ = 16 states (0–15).
31. The basic sequential circuit element is
A) AND gate
B) Flip-flop
C) Decoder
D) Multiplexer
Answer: B
Explanation: Flip-flops store state, forming sequential circuits.
32. Gray code is used to
A) Simplify addition
B) Avoid errors during bit transitions
C) Speed up circuits
D) Encrypt data
Answer: B
Explanation: Gray code changes only one bit at a time to reduce errors.
33. The ASCII code represents
A) Numbers only
B) Characters and symbols
C) Colors
D) Binary codes
Answer: B
Explanation: ASCII represents letters, numbers, and symbols in binary.
34. The logic family known for low power consumption is
A) TTL
B) ECL
C) CMOS
D) RTL
Answer: C
Explanation: CMOS uses complementary transistors with very low power usage.
35. Propagation delay is
A) Power loss in circuit
B) Time taken for output to respond to input
C) Time between clock pulses
D) Data transfer time
Answer: B
Explanation: Propagation delay = delay between input change and output response.
36. In binary addition, 1 + 1 =
A) 1
B) 10
C) 11
D) 0
Answer: B
Explanation: 1 + 1 = 10₂ (0 with carry 1).
37. The output of a half adder is same as
A) XOR and AND
B) OR and AND
C) NAND and NOR
D) XOR and OR
Answer: A
Explanation: Half adder uses XOR for sum and AND for carry.
38. The main disadvantage of TTL logic is
A) High speed
B) High power consumption
C) Low cost
D) Noise immunity
Answer: B
Explanation: TTL consumes more power than CMOS.
39. A logic circuit that performs addition of two binary numbers is called
A) Subtractor
B) Adder
C) Decoder
D) Encoder
Answer: B
Explanation: Adder performs binary addition.
40. The output of a D flip-flop is
A) Complement of input
B) Same as input
C) Always 1
D) Always 0
Answer: B
Explanation: Output follows the D input after clock trigger.
41. The output of SR flip-flop is invalid when
A) S=0, R=0
B) S=1, R=0
C) S=0, R=1
D) S=1, R=1
Answer: D
Explanation: Both inputs high cause indeterminate state.
42. A ripple counter is also known as
A) Synchronous counter
B) Asynchronous counter
C) Binary multiplier
D) Ring counter
Answer: B
Explanation: Ripple counter works asynchronously — clock ripples through stages.
43. ROM stands for
A) Read Out Memory
B) Read Only Memory
C) Random Only Memory
D) Ready On Memory
Answer: B
Explanation: ROM stores data permanently, only readable.
44. The operation of RAM is
A) Read only
B) Write only
C) Read and Write
D) None
Answer: C
Explanation: RAM allows both read and write operations.
45. The basic difference between combinational and sequential circuit is
A) Logic gates used
B) Feedback
C) Number of inputs
D) Voltage level
Answer: B
Explanation: Sequential circuits use feedback and memory; combinational don’t.
46. The binary equivalent of hexadecimal A3 is
A) 10100011
B) 11001100
C) 10010011
D) 11100111
Answer: A
Explanation: A = 1010, 3 = 0011 → A3 = 10100011₂.
47. Which device is used to synchronize data between two systems?
A) Counter
B) Flip-flop
C) Register
D) Latch
Answer: B
Explanation: Flip-flops are clocked devices used for synchronization.
48. The NOT gate is also called
A) Inverter
B) Buffer
C) Driver
D) Follower
Answer: A
Explanation: NOT gate inverts the input logic level.
49. A BCD code represents
A) Decimal digits in binary
B) Binary digits in decimal
C) ASCII characters
D) Hexadecimal numbers
Answer: A
Explanation: BCD = Binary Coded Decimal (each decimal digit represented in 4 bits).
50. The output of XOR gate is 1 only when
A) Both inputs are 0
B) Both inputs are 1
C) Inputs are different
D) Inputs are same
Answer: C
Explanation: XOR outputs 1 when inputs differ.
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