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Digital Logic (DL) Important Questions

Binary systems

  1. Differentiate between Analog and Digital system. What are the advantages of a digital system?
  2. What is a digital computer? Explain the working principle of digital computers with a block diagram.
  3. What do you mean by the Gray code? What are its application?
  4. What is decimal code? Differentiate between BCD and excess-3 code.
  5.  Explain the error detection code with an example.
  6. Convert the following decimal numbers to the indicated bases.
    1. 7562.45 to octal
    2. 1938.257 to hexadecimal
    3. 175.175 to binary
  7. Convert the following hexadecimal number to decimal and octal numbers
    1. 0FFF
    2. 3FFF
  8. Subtract (1010100 – 1000100) using 1’s complement.
  9. Using 2’s complement, subtract (1000100-1010100).

Boolean Algebra and Logic Gates

  1. State and prove commutative laws, associative laws and distributive law using logic gate and truth table.
  2. What do you mean by Universal gate?   Show that both NAND gate and NOR gate are universal gates.
  3.  State and prove De-Morgan’s theorem 1st and 2nd with logic gates and truth table.
  4. Explain the duality theorem with example.Simplify the following Boolean functions to a minimum number of literals: xy + x′z + yz
  5. Prove the Boolean expression: AB + AB′C + A′BC = AB + AC + BC
  6. What is a logic gate? What are the types of basic gate.
  7. Draw a logic gates that implement the following:
F = AB + C D' + B' C
F = (A + B) (B' + C) (C' + D + E)
  1. Define integrated circuit. What are the characteristics that describe the performance of IC digital logic families?

Simplification of Boolean Functions

  1. Explain the K-map with three variables. Reduce the following function using k-map
F = B'D + A'BC' + AB'C + ABC'
  1. What do you mean by K-map?Write a procedure to reduce K-maps.
  2. How do you express the Boolean function as a product of maxterms? Express the Boolean function F = xy + x′z in a product of maxterm form.
  3. Simplify the Boolean function: F(X,Y,Z) = ∑(0, 2, 4, 5, 6) using three variable K-map.
  4. Simplify the Boolean function F = A′B′C′ + B′CD′ + A′BCD′ + AB′C′ using K-map.
  5. Why don’t care conditions are used? Simplify F(w, x, y, z) = ∑(1, 3, 7, 11, 15) which has the don’t care conditions d(w, x, y, z) = ∑(0, 2, 5).
  6. Describe the forms of Boolean algebra with example.Define maxterms and minterms.

Combinational Logic

  1. What is combinational logic? Design a half adder logic using only NAND gates.
  2. Design the full subtractor circuit with using Decoder and explain the working principle.
  3. What do you mean by full adder and full subtractor?  Design a half subtractor using only NOR gates.
  4. Design a full adder with truth table and logic gates.
  5. Explain parity generator and checker with suitable example.
  6. Describe BCD to excess-3 Code Conversion with truth table and logic diagram.
  7. What is combinational logic? What are its important features. Design a half adder logic using only NOR gate.

Combination Logic with MSI and LSI

  1. What is binary parallel adder? Describe 4-bit binary parallel adder with logic diagram.
  2. What is magnitude comparator? Design a logic circuit for 4 bit magnitude comparator and explain it.
  3. What is decimal adder? Explain it with truth table and logic diagram.
  4. What do you mean by decoder? Design a 3 to 8 line decoder using 2 to 4 line decoder and explain it.
  5. Differentiate between a MUX and a DEMUX. Draw a logic circuit of 8*1 multiplexer.
  6. Implement a full-adder circuit with a decoder and two OR gates.
  7. What is encoder? Design a 3 to 8 line decoder using two 2 to 4 line decoder and explain it.
  8. What is demultiplexer? Draw its block diagram and explain its working principle.
  9. Using a decoder and external gates, design the combinational circuit defined by the following three Boolean functions:
F1 = x′y′z + xz′
F2 = x′yz′ + xy′
F3 = xyz′ + xy
  1. Implement the following function Σ (0,1,3,4,8,9,10) using
    1. Decoder
    2. Multiplexer
    3. PLA
  1. Differentiate between PAL and PLA. A combinational circuit is defined by the functions:
F1(A, B,C) = ∑(3, 5, 6, 7)
F2(A, B,C) = ∑(0, 2, 4, 7)

Implement the circuit with a PLA having three inputs, four product terms, and two outputs.

  1.  Explain the PLA with the block diagram. Describe types of ROM in brief.

Synchronous and Asynchronous Sequential Logic

  1. Differentiate between combinational logic and sequential logic. List some applications of sequential logic.
  2. What is flip-flop? Mention the application of flip-flop.
  3. List the types of flip-flop.Describe the clocked RS flip-flop.
  4. How does a J-K flip flop differs from an S-R flip flop in its basic operations? Explain.
  5. Differentiate between Synchronous Sequential circuit and Asynchronous Sequential Circuit. What do you mean by D-flip-flop?
  6. What is master-slave flip-flop? Explain master slave J-K flipflop.
  7. What do you mean by triggering of flip flop? Define state table, state diagram and state equation.
  8. Explain design Procedure of clocked Sequential Circuit with suitable example.
  9. What is State reduction table? How JK flip flop can convert into a D-flip flop?

Registers and counters   

  1. What do you mean by ripple counters? Explain the 4 bit ripple counter and also draw a timing diagram.
  2. What do you mean by shift registers? Mention the difference types of shift register.
  3. What is the difference between a serial and parallel transfer? Explain how to convert serial data to parallel and parallel data to serial. What type of register is needed?
  4. What do you mean by asynchronous counter? Design the mod-6 asynchronous counter and explain with truth table.
  5. What do you mean by synchronous counter?Design a 3 bit synchronous counter and explain it.
  6. What do you mean by counter? Design a counter as shown in the state diagram below


  1. Design the 4-bit synchronous up/down counter with timing diagram, logic diagram and truth table.
  2. Design a 3 bit synchronous counter and explain it.
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