Questions
Syllabus
Notes
Qns. By Syllabus

Semester

Subject

Year

Tribhuwan University

Institute of Science and Technology

2077

Bachelor Level / First Year / First Semester / Science

Bachelors in Information Technology (BIT103)

(Digital Logic)

Full Marks: 60

Pass Marks: 24

Time: 3 Hours

Candidates are required to give their answers in their own words as for as practicable.

The figures in the margin indicate full marks.

Section A

Long Answers Questions

Attempt any TWO questions.
[2*10=20]
1.
What is decoder? Design BCD to Decimal Decoder with truth table and logic diagram.[10]

BCD to Decimal Decoder

A decoder is a combinational logic circuit that converts an n-bit binary input code into a maximum of 2n2^n unique output lines, where only one output is active at a time.

Definition and Overview

A decoder accepts a set of inputs representing a binary number and activates exactly one of the output lines corresponding to that number. It is essentially a min-term generator.

A BCD to Decimal Decoder (also called a 4-line to 10-line decoder) takes a 4-bit BCD input (A, B, C, D) and activates one of 10 output lines (Y₀ to Y₉) representing decimal digits 0 through 9.

  • It has 4 inputs: A (MSB), B, C, D (LSB)
  • It has 10 outputs: Y₀, Y₁, Y₂, Y₃, Y₄, Y₅, Y₆, Y₇, Y₈, Y₉
  • Only valid BCD combinations (0000 to 1001) are used
  • Combinations 1010 to 1111 are invalid (don't care) in BCD

Truth Table

A B C D Y₀ Y₁ Y₂ Y₃ Y₄ Y₅ Y₆ Y₇ Y₈ Y₉
0 0 0 0 1 0 0 0 0 0 0 0 0 0
0 0 0 1 0 1 0 0 0 0 0 0 0 0
0 0 1 0 0 0 1 0 0 0 0 0 0 0
0 0 1 1 0 0 0 1 0 0 0 0 0 0
0 1 0 0 0 0 0 0 1 0 0 0 0 0
0 1 0 1 0 0 0 0 0 1 0 0 0 0
0 1 1 0 0 0 0 0 0 0 1 0 0 0
0 1 1 1 0 0 0 0 0 0 0 1 0 0
1 0 0 0 0 0 0 0 0 0 0 0 1 0
1 0 0 1 0 0 0 0 0 0 0 0 0 1

Logic Expressions (Output Equations)

Each output is a minterm of the 4 input variables:

  • Y0=A′B′C′D′Y_0 = A'B'C'D'
  • Y1=A′B′C′DY_1 = A'B'C'D
  • Y2=A′B′CD′Y_2 = A'B'CD'
  • Y3=A′B′CDY_3 = A'B'CD
  • Y4=A′BC′D′Y_4 = A'BC'D'
  • Y5=A′BC′DY_5 = A'BC'D
  • Y6=A′BCD′Y_6 = A'BCD'
  • Y7=A′BCDY_7 = A'BCD
  • Y8=AB′C′D′Y_8 = AB'C'D'
  • Y9=AB′C′DY_9 = AB'C'D

Logic Diagram

image

Conclusion

The BCD to Decimal decoder is widely used in digital displays, address decoding, and memory selection circuits. IC 7442 is a standard TTL BCD to Decimal decoder available commercially.

2.
Design a combinational circuit whose input is a four-bit number and output is 2's complement of the input number.[10]
3.
What is ring counter? Explain ring counter with diagram, timing sequence and timing diagram.[10]
Section B

Short Answers Questions

Attempt any Eight questions.
[8*5=40]
4.
Convert (1011.110) into decimal and hexadecimal. [5]
5.
Substract (1011.11- 1010.10) using 2's and l's complement. [5]
6.
Express the given function in sum of minterms. F = y'z + wxy' + wzx' + w'x'z' [5]
7.
Simplify the Boolean function using don't care conditions d in sum of products form. and product of sums form F(A,B,C,D)=π(1,3,7,8,12) and πd(5,10,13,14)F(A, B, C, D) = \pi(1, 3, 7, 8, 12) \text{ and } \pi d(5, 10, 13, 14) [5]
8.
Implement a full subtractor with two half subtractors and one OR Gate. [5]
9.
Define SR latch with logic diagram and truth table. [5]
10.
Compare the logic of synchronous counter and ripple counter. [5]
11.
Define shift register with its types. [5]
12.
Write short notes on: a.) Parity generator Write short notes on: a.) State Diagram [2.5+2.5]