Page 36 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Truth Table

Inputs Outputs
A B C in Sum Carry
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
Binary substraction circuits
Substraction is a mathematical operation in which one integer number is deducted from another to obtain the
equivalent quantity. The number from which other number is to be deducted is called as minuend and the number
subtracted from the minuend is called subtrahend.

Half Subtractors
A half subtractor is a multiple output combinational logic network that does the subtraction of two bits of binary
data. It has input variables and two output variables. Two inputs are corresponding to two input bits and two output
variables corresponds to the difference bit and borrow bit.
The binary subtraction is also performed by the Ex-OR gate with additional circuitry to perform the borrow opera-
tion. Thus, a half subtractor is designed by an Ex-OR gate including AND gate with A input complemented before
fed to the gate.
In case of multi-digit subtraction, subtraction between the two digits must be performed along with borrow of the
previous digit subtraction, and hence a subtractor needs to have three inputs. Therefore, a half subtractor has
limited applications and strictly it is not used in practice.

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CITS : E & H - Electronics Mechanic - Lesson 9 - 29