Page 55 - Digital Electronics by harish
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Figure : Logic diagram
SUM = A ⊕ B ⊕ C
CARRY = A B + (A ⊕ B) C
The first Half adder HA1 is used to add A and B. Its SUM output and C are added by the
second Half adder HA2. The SUM output of HA2 is the final SUM. The carry outputs of
HA1 and HA2 are ORed to get the final CARRY output.
2.4 Half subtractor
Half subtractor subtracts one binary bit from another binary bit at a time. It has two inputs A
and B, and two outputs DIFFERENCE and BORROW.
Figure : Block diagram of half subtractor
As per the laws of binary subtraction, we get,
When A= 0 and B = 0
0 - 0 ===> Borrow = 0, Difference = 0
When A= 0 and B = 1
0 - 1 ===> Borrow = 1, Difference = 1
When A= 1 and B = 0
1 - 0 ===> Borrow = 0, Difference = 1
When A= 1 and B = 1
1 - 1 ===> Borrow = 0, Difference = 0
The logic diagram, truth table and logic equations for DIFFERENCE and BORROW are
given in figure.
Borrow = . B
Figure : Logic diagram of half subtractor
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