Basic Digital Techniques & Applications - PART 6

07:18:00 Nandan Bhalwankar 0 Comments

Half Subtractors

As the name indicates, it is used for subtraction of numbers. The half subtractor subtracts two single binary digits A and B. It has two outputs, Difference (D) and Borrow(B). Since the circuit subtracts the two digits, it is called as ‘Half Subtractor’ circuit.    

Logic Diagram of Half Subtractor circuit:

 

Truth table of Half Subtractor circuit:

If we look at the truth table, row numbers 1, 3 and 4 look obvious. However, for second row, when inputs A=0 and B=1, the outputs are D=1 and B=1.

d) Full Subtractors:

The full subtractors subtract the three one bit binary numbers A, B and C and produceDifference (D) and Borrow(B) as the output. The full subtractor is basically a combination of half subtractors.

Truth table of Full Subtractors circuit:

For the last row in above truth table, subtraction and borrow are as mentioned below:

   A=1

- B=1

__________________

Difference=0   (Borrow=0)

-Bin=1

__________________

Final Difference=1 (Borrow=1) [See the final row]

K-map simplification of Full Subtractor circuit:

Logic Diagram of Full Subtractor circuit: