COMPUTER SCIENCE ILLUMIN.-TEXT
COMPUTER SCIENCE ILLUMIN.-TEXT
7th Edition
ISBN: 9781284156010
Author: Dale
Publisher: Jones & Barlett
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Chapter 4, Problem 72E

Explanation of Solution

Circuit diagram for combining the two full adders and its equivalent truth table

Full adder:

  • The full adder is an electric circuit which is used to perform the addition of three binary digits.
  • It takes the inputs of two binary digits and another input is the carry-in value.
  • It produces the output as sum and carry-out.
  • Logic diagram for the full adder takes three inputs A, B, and Carry-in and produces the output Sum and Carry-out.

COMPUTER SCIENCE ILLUMIN.-TEXT, Chapter 4, Problem 72E , additional homework tip  1

Circuit diagram for combining the two full adders:

  • The first full adder takes the input as B, D, Carry-in (assume 0) and produces the output Z and Carry.
  • The second full adder takes the input as A, C, and output of the first full adder “Carry” and produces the output as Y and X.
  • Diagrammatic representation after combining two full adders is shown below:

COMPUTER SCIENCE ILLUMIN.-TEXT, Chapter 4, Problem 72E , additional homework tip  2

  • Circuit diagram:
  • Combine the logic diagram of the two full adders that adds two two-bit binary numbers. The circuit diagram is shown below:

COMPUTER SCIENCE ILLUMIN.-TEXT, Chapter 4, Problem 72E , additional homework tip  3

Truth table explanation:

  • From the above circuit diagram:
    • Boolean expression for Z:
      • First, the inputs B and D are passed to XOR gate to produce the output as BD.
      • Next, pass the output of first XOR gate “BD” and “Carry-in” as the input of second XOR gate to produce the output as Z = ((BD)Carry-in).
    • Boolean expression for Carry:
      • Next, pass the same inputs B and D from the first XOR gate to AND gate to produce the output as BD.
      • Next, pass again the output of first XOR gate “BD” and “Carry-in” as the input of second AND gate to produce the output as ((BD)Carry-in).
      • Finally, pass the output of first AND gate “BD” and the output of second AND gate “((BD)Carry-in)” as the input of OR gate to produce the output as Carry = (BD) + ((BD)Carry-in).
    • Boolean expression for Y:
      • First, the inputs A and C are passed to XOR gate to produce the output as AC.
      • Next, pass the output of first XOR gate “AC” and “Carry” as the input of second XOR gate to produce the output as Y = ((AC)Carry).
    • Boolean expression for X:
      • Next, pass the same inputs A and C from the first XOR gate to AND gate to produce the output as AC.
      • Next, pass again the output of first XOR gate “AC” and “Carry” as the input of second AND gate to produce the output as ((AC)Carry).
      • Finally, pass the output of first AND gate “AC” and the output of second AND gate “((AC)Carry)” as the input of OR gate to produce the output as X = (AC) + ((AC)Carry).
  • Therefore, the Boolean expression for combining two full adders produce X, Y, and Z.

X = (AC) + ((AC)Carry)Y = ((AC)Carry)Z = ((BD)Carry-in)

Truth table for combining the two full adders:

ABCDCarry-inBDACZCarryYX
00000000000
00010101000
00100010010
00110111010
01000101000
01010000110
01100111010
01110010101
10000010010
10010111010
10100000001
10110101001
11000111010
11010010101
11100101001
11110000111

For example:

  • If the inputs are A as 0, B as 0, C as 1, D as 0, and Carry-in as 0, then the full adder adds the two two-bit binary numbers as shown below:

COMPUTER SCIENCE ILLUMIN.-TEXT, Chapter 4, Problem 72E , additional homework tip  4

  • Then, it produces the output of X, Y, Z, and Carry. Truth table is shown below:
ABCDCarry-inBDACZCarryYX
00000000000
00010101000
00100010010
00110111010
01000101000
01010000110
01100111010
01110010101
10000010010
10010111010
10100000001
10110101001
11000111010
11010010101
11100101001
11110000111

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