Digital Logic Design MCQs with Answers: Top 50 DLD MCQs

Introduction

Digital Logic Design (DLD MCQS) forms the foundational backbone of computer architecture, digital electronics, and microprocessors. Students and aspirants preparing for PPSC Lecturer Computer Science, FPSC, NTS, and engineering recruitment tests frequently encounter technical questions regarding logic gates, Boolean algebra, Karnaugh maps (K-maps), combinational circuits, and sequential circuits. This comprehensive guide features 50 high-yield Digital Logic Design MCQs with verified answers and detailed explanations to strengthen your core technical concepts.

Section 1: Number Systems & Boolean Algebra

Q1. Which number system uses digits from 0 to 7? A) Binary

B) Octal

C) Decimal

D) Hexadecimal

Correct Answer: B) Octal

Explanation: The octal number system is a base-8 system that utilizes eight distinct digits, ranging from 0 through 7.

Q2. What is the hexadecimal equivalent of the binary number 1101? A) A

B) B

C) C

D) D

Correct Answer: D) D

Explanation: In binary, 1101 equals $8 + 4 + 0 + 1 = 13$ 8+4+0+1=13, which corresponds to the hexadecimal digit ‘D’ (where A=10, B=11, C=12, D=13).

Q3. According to Boolean algebra, what is the simplification of the expression A+ABA + ABA+AB? A) $B$ B

B) $A$ A

C) $AB$ AB

D) $1$ 1

Correct Answer: B) $A$ A

Explanation: By the absorption law of Boolean algebra, $A + AB = A(1 + B) = A(1) = A$ A+AB=A(1+B)=A(1)=A.

Q4. What does De Morgan’s first law state? A) $\overline{A + B} = \bar{A} \cdot \bar{B}$ A+B=Aˉ⋅Bˉ

B) $\overline{A \cdot B} = \bar{A} + \bar{B}$ A⋅B=Aˉ+Bˉ

C) $A + B = B + A$ A+B=B+A

D) $A \cdot (B + C) = AB + AC$ A⋅(B+C)=AB+AC

Correct Answer: A) $\overline{A + B} = \bar{A} \cdot \bar{B}$ A+B=Aˉ⋅Bˉ

Explanation: De Morgan’s first theorem states that the complement of a sum is equal to the product of individual complements.

Q5. Which logic gate outputs a HIGH (1) only when all its inputs are HIGH (1)? A) OR gate

B) AND gate

C) NOR gate

D) XOR gate

Correct Answer: B) AND gate

Explanation: An AND gate performs logical multiplication; its output is 1 if and only if every input is 1.

Q6. Which logic gate is known as the universal gate because any Boolean function can be implemented using it alone? A) AND gate

B) OR gate

C) NAND gate

D) Buffer

Correct Answer: C) NAND gate

Explanation: Both NAND and NOR gates are universal gates capable of implementing any digital logic expression.

Q7. What is the output of an Exclusive-OR (XOR) gate when both inputs are identical (both 0 or both 1)? A) 0

B) 1

C) High Impedance

D) Undefined

Correct Answer: A) 0

Explanation: An XOR gate outputs 1 only when inputs differ. When inputs are identical, the output is 0.

Q8. What is the primary purpose of a Karnaugh Map (K-map)? A) To convert analog signals to digital

B) To simplify Boolean algebra expressions

C) To store binary data

D) To generate clock pulses

Correct Answer: B) To simplify Boolean algebra expressions

Explanation: A K-map is a graphical tool used to minimize Boolean expressions without manual algebraic calculation.

Q9. In a standard Sum of Products (SOP) expression, what does a binary ‘0’ represent for variable AAA? A) $A$ A

B) $\bar{A}$ Aˉ (Complement)

C) Ground

D) Undefined

Correct Answer: B) $\bar{A}$ Aˉ (Complement)

Explanation: In SOP representation, literal 0 represents the complemented variable ( $\bar{A}$ Aˉ), while 1 represents the uncomplemented variable ( $A$ A).

Q10. How many rows are present in the truth table of a Boolean function with 4 input variables? A) 8

B) 12

C) 16

D) 32

Correct Answer: C) 16

Explanation: The number of truth table rows equals $2^n$ 2n, where $n$ n is the number of inputs ( $2^4 = 16$ 24=16).

Section 2: Combinational Logic Circuits

Q11. What combinational circuit adds two single-bit binary numbers and produces a sum and a carry? A) Half Adder

B) Full Adder

C) Half Subtractor

D) Multiplexer

Correct Answer: A) Half Adder

Explanation: A half adder handles two inputs ( $A$ A and $B$ B) and generates Sum and Carry outputs, but lacks a provision for input carry.

Q12. How many inputs and outputs does a Full Adder possess? A) 2 inputs, 2 outputs

B) 3 inputs, 2 outputs

C) 3 inputs, 3 outputs

D) 4 inputs, 2 outputs

Correct Answer: B) 3 inputs, 2 outputs

Explanation: A full adder takes three inputs ( $A$ A, $B$ B, and Carry-in $C_{in}$ Cin) and produces two outputs (Sum and Carry-out).

Q13. What circuit converts binary information from nnn input lines to 2n2^n2n unique output lines? A) Multiplexer

B) Decoder

C) Encoder

D) Demultiplexer

Correct Answer: B) Decoder

Explanation: A decoder activates a single output line corresponding to the binary code applied to its $n$ n input lines.

Q14. What combinational device selects data from one of many input lines and directs it to a single output line? A) Decoder

B) Multiplexer (MUX)

C) Demultiplexer

D) Encoder

Correct Answer: B) Multiplexer (MUX)

Explanation: A multiplexer acts as a digital switch, routing one of several data inputs to a single output based on select lines.

Q15. What device performs the reverse operation of a Multiplexer, taking one input and routing it to one of 2n2^n2n outputs? A) Encoder

B) Demultiplexer (DEMUX)

C) Comparator

D) Adder

Correct Answer: B) Demultiplexer (DEMUX)

Explanation: A demultiplexer channels a single input signal to one of multiple destinations determined by select inputs.

Q16. Which combinational circuit detects whether two binary numbers are equal? A) Decoder

B) Magnitude Comparator

C) Encoder

D) Multiplexer

Correct Answer: B) Magnitude Comparator

Explanation: A digital comparator compares two binary numbers ( $A$ A and $B$ B) and outputs indicators for $A > B$ A>B, $A < B$ A<B, and $A = B$ A=B.

Q17. How many select lines are required for a Multiplexer with 8 input lines (8×18\times18×1 MUX)? A) 2

B) 3

C) 4

D) 8

Correct Answer: B) 3

Explanation: Since $2^n = 8$ 2n=8, the number of select lines $n = 3$ n=3 ( $2^3 = 8$ 23=8).

Q18. What circuit compresses 2n2^n2n input lines into nnn output lines representing a binary code? A) Decoder

B) Encoder

C) Multiplexer

D) Register

Correct Answer: B) Encoder

Explanation: An encoder performs the inverse function of a decoder, translating active input lines into an equivalent binary code.

Q19. What type of adder circuit can add multi-bit binary numbers by cascading multiple full adders? A) Half Adder

B) Ripple Carry Adder

C) Serial Adder

D) Multiplexed Adder

Correct Answer: B) Ripple Carry Adder

Explanation: A ripple carry adder chains full adders together, where the carry-out of each block ripples into the carry-in of the next.

Q20. What logic gate combination implements an Exclusive-NOR (XNOR) operation? A) XOR followed by NOT

B) AND followed by OR

C) NOR followed by AND

D) OR followed by NAND

Correct Answer: A) XOR followed by NOT

Explanation: An XNOR gate outputs 1 when both inputs match. It is constructed by passing an XOR output through an inverter (NOT gate).

Section 3: Sequential Circuits & Flip-Flops

Q21. What primary characteristic distinguishes sequential circuits from combinational circuits? A) Use of logic gates

B) Presence of feedback paths and memory elements

C) Operating speed

D) Use of binary numbers

Correct Answer: B) Presence of feedback paths and memory elements

Explanation: Sequential circuits depend on current inputs and past states stored in internal memory elements (flip-flops).

Q22. What is the fundamental building block of sequential logic circuits? A) Logic Gate

B) Multiplexer

C) Flip-Flop (Latch)

D) Decoder

Correct Answer: C) Flip-Flop (Latch)

Explanation: Flip-flops and latches are bistable multivibrators capable of storing one bit of binary data.

Q23. In an SR Flip-Flop, what condition represents the forbidden or invalid input state? A) $S=0, R=0$ S=0,R=0

B) $S=0, R=1$ S=0,R=1

C) $S=1, R=0$ S=1,R=0

D) $S=1, R=1$ S=1,R=1

Correct Answer: D) $S=1, R=1$ S=1,R=1

Explanation: Setting both Set ( $S$ S) and Reset ( $R$ R) to 1 simultaneously creates an ambiguous, unstable state in basic SR latches.

Q24. Which flip-flop eliminates the invalid state of the SR flip-flop by tying RRR to the inverted SSS input? A) JK Flip-Flop

B) D Flip-Flop

C) T Flip-Flop

D) Master-Slave Flip-Flop

Correct Answer: B) D Flip-Flop

Explanation: The Data (D) flip-flop takes a single input, ensuring that the output directly follows the input on the clock edge.

Q25. What happens in a JK Flip-Flop when both J=1J=1J=1 and K=1K=1K=1 during a clock pulse? A) Output resets to 0

B) Output sets to 1

C) Output toggles (inverts state)

D) Output remains unchanged

Correct Answer: C) Output toggles (inverts state)

Explanation: The toggle condition ( $J=1, K=1$ J=1,K=1) in a JK flip-flop reverses the current output state on each clock cycle.

Q26. What does the ‘T’ stand for in a T Flip-Flop? A) Trigger

B) Toggle

C) Transfer

D) Timing

Correct Answer: B) Toggle

Explanation: A T flip-flop toggles its output state whenever its Toggle input is high (1) on the active clock edge.

Q27. What sequential circuit issue is resolved by using a Master-Slave Flip-Flop configuration? A) Propagation delay

B) Race around condition

C) Power consumption

D) Bit overflow

Correct Answer: B) Race around condition

Explanation: The race-around condition occurs in JK flip-flops when pulses are too long; master-slave architecture prevents this by cascading two latches.

Q28. What type of sequential circuit changes its state based on internal clock pulses? A) Asynchronous sequential circuit

B) Synchronous sequential circuit

C) Combinational circuit

D) Unclocked circuit

Correct Answer: B) Synchronous sequential circuit

Explanation: Synchronous circuits synchronize state transitions using a global clock signal.

Q29. What is an asynchronous sequential circuit also commonly referred to as? A) Clocked circuit

B) Event-driven circuit

C) Synchronized system

D) Pipelined system

Correct Answer: B) Event-driven circuit

Explanation: Asynchronous circuits respond immediately to input changes without waiting for a clock signal.

Q30. Which finite state machine model produces outputs that depend solely on current internal flip-flop states? A) Mealy Machine

B) Moore Machine

C) Combinational Machine

D) Asynchronous Machine

Correct Answer: B) Moore Machine

Explanation: In a Moore machine, outputs are a function of the current state only, whereas in a Mealy machine, outputs depend on both state and inputs.

Section 4: Registers, Counters & Memory

Q31. What digital circuit consists of an array of flip-flops used to store and shift binary data? A) Counter

B) Shift Register

C) Multiplexer

D) Adder

Correct Answer: B) Shift Register

Explanation: Shift registers store data and shift bits left or right across flip-flop stages upon clock pulses.

Q32. A register that accepts data input simultaneously across all bits and releases it similarly is called: A) Serial-In Serial-Out (SISO)

B) Parallel-In Parallel-Out (PIPO)

C) Serial-In Parallel-Out (SIPO)

D) Ring Counter

Correct Answer: B) Parallel-In Parallel-Out (PIPO)

Explanation: PIPO registers load and output all binary bits simultaneously.

Q33. What sequential circuit counts the number of clock pulses applied to its input? A) Register

B) Counter

C) Decoder

D) Comparator

Correct Answer: B) Counter

Explanation: Counters track event occurrences by cycling through a predefined sequence of binary states.

Q34. What type of counter propagates clock signals from one flip-flop output to the clock input of the next? A) Synchronous Counter

B) Ripple (Asynchronous) Counter

C) Ring Counter

D) Johnson Counter

Correct Answer: B) Ripple (Asynchronous) Counter

Explanation: In ripple counters, flip-flops do not trigger simultaneously; each stage triggers from the previous stage’s output.

Q35. What is the primary advantage of synchronous counters over ripple counters? A) Simpler circuitry

B) Lower power consumption

C) Elimination of propagation delay accumulation

D) Fewer flip-flops required

Correct Answer: C) Elimination of propagation delay accumulation

Explanation: Synchronous counters apply clock pulses to all flip-flops simultaneously, eliminating cumulative ripple delay.

Q36. What is the maximum count modulus of a binary ripple counter constructed with 4 flip-flops? A) 8

B) 10

C) 16

D) 32

Correct Answer: C) 16

Explanation: A 4-bit counter has $2^4 = 16$ 24=16 distinct states (0 through 15).

Q37. What type of counter counts in a specific binary code sequence, such as Binary Coded Decimal (BCD)? A) Decade Counter

B) Ripple Counter

C) Up/Down Counter

D) Shift Counter

Correct Answer: A) Decade Counter

Explanation: A decade counter cycles through 10 states (0 to 9) before resetting, making it ideal for decimal counting.

Q38. What memory technology retains data as long as power is supplied, using microscopic capacitors that require periodic refreshing? A) SRAM

B) DRAM

C) EEPROM

D) Flash Memory

Correct Answer: B) DRAM

Explanation: Dynamic RAM (DRAM) stores bits in capacitors that leak charge, necessitating constant refresh cycles.

Q39. Which semiconductor memory type is faster and does not require refreshing, but occupies more physical chip space? A) DRAM

B) SRAM (Static RAM)

C) Magnetic Tape

D) Optical Disk

Correct Answer: B) SRAM (Static RAM)

Explanation: Static RAM uses flip-flop circuitry to store each bit, providing high speed without refresh requirements.

Q40. What programmable memory device can be erased using ultraviolet (UV) light exposure? A) PROM

B) EPROM

C) EEPROM

D) RAM

Correct Answer: B) EPROM

Explanation: Erasable Programmable Read-Only Memory (EPROM) features a quartz window allowing UV light to clear stored data.

Section 5: Advanced Logic Families & Timing (41-50)

Q41. What does the acronym TTL stand for in digital electronics logic families? A) Transistor-Transistor Logic

B) Twin-Transistor Latency

C) Timing Trigger Logic

D) Transfer Transistor Line

Correct Answer: A) Transistor-Transistor Logic

Explanation: TTL is a popular digital logic family built using bipolar junction transistors (BJTs).

Q42. Which logic family is renowned for extremely low power consumption and high noise immunity using field-effect transistors? A) TTL

B) ECL

C) CMOS (Complementary Metal-Oxide-Semiconductor)

D) RTL

Correct Answer: C) CMOS (Complementary Metal-Oxide-Semiconductor)

Explanation: CMOS technology dominates modern processor and integrated circuit design due to near-zero static power dissipation.

Q43. What term describes the time interval between the application of an input signal and its resulting output change? A) Rise time

B) Propagation Delay

C) Clock skew

D) Setup time

Correct Answer: B) Propagation Delay

Explanation: Propagation delay measures the transit time of signals passing through logic gates or circuits.

Q44. What is Fan-out in digital integrated circuits? A) Maximum operating frequency

B) Number of standard load inputs a gate output can drive without performance loss

C) Power dissipation rating

D) Heat dissipation capacity

Correct Answer: B) Number of standard load inputs a gate output can drive without performance loss

Explanation: Fan-out specifies the load-driving capacity of a logic gate output.

Q45. What digital circuit component converts physical analog sensor voltages into equivalent digital binary numbers? A) DAC (Digital-to-Analog Converter)

B) ADC (Analog-to-Digital Converter)

C) Multiplexer

D) Oscillator

Correct Answer: B) ADC (Analog-to-Digital Converter)

Explanation: ADCs bridge real-world analog signals (temperature, sound) with digital computer processing systems.

Q46. What type of programmable logic device consists of a programmable AND array followed by a fixed OR array? A) PAL (Programmable Array Logic)

B) PLA (Programmable Logic Array)

C) FPGA

D) ROM

Correct Answer: A) PAL (Programmable Array Logic)

Explanation: PAL devices feature programmable AND gates and fixed OR gates, making them simpler and faster than PLAs.

Q47. What advanced integrated circuit contains an array of programmable logic blocks and interconnects configurable by the user? A) Logic Gate

B) FPGA (Field Programmable Gate Array)

C) Half Adder

D) Encoder

Correct Answer: B) FPGA (Field Programmable Gate Array)

Explanation: FPGAs offer reconfigurable hardware logic fabrics suited for custom digital signal processing and prototyping.

Q48. In timing analysis, what is “Setup Time”? A) Time data must remain stable after the clock edge

B) Minimum time data must be stable before the active clock edge

C) Total propagation delay

D) Clock cycle period

Correct Answer: B) Minimum time data must be stable before the active clock edge

Explanation: Setup time ensures reliable data latching by requiring input signals to settle prior to clock triggering.

Q49. What timing parameter specifies how long data must remain unchanged after the active clock transition? A) Setup time

B) Hold time

C) Propagation delay

D) Rise time

Correct Answer: B) Hold time

Explanation: Hold time prevents premature input changes from corrupting data during flip-flop clock triggering.

Q50. What type of multivibrator circuit has two stable states and requires external trigger pulses to switch between them? A) Astable Multivibrator

B) Monostable Multivibrator

C) Bistable Multivibrator (Flip-Flop)

D) Free-running Oscillator

Correct Answer: C) Bistable Multivibrator (Flip-Flop)

Explanation: Bistable multivibrators hold their state indefinitely until triggered to change, serving as the basis for digital memory.

Conclusion

Mastering these 50 Digital Logic Design (DLD) MCQs provides a rock-solid grasp of combinational and sequential circuit principles, Boolean logic simplification, and computer hardware fundamentals. Consistent practice of these technical questions will ensure top-tier preparation for upcoming PPSC, FPSC, NTS, and computer science examinations.

Related Subject-wise MCQs:

Section 1: Number Systems & Boolean Algebra

Section 2: Combinational Logic Circuits

Section 3: Sequential Circuits & Flip-Flops

Section 4: Registers, Counters & Memory

Section 5: Advanced Logic Families & Timing (41-50)

Conclusion

Related Subject-wise MCQs:

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