Question

Design bcd adder diagram using 7486

Answer 1

A bcd adder is used to add 2 bcd number and produces a bcd number as output.It cannot be more than 9. If the sum of 2 bcd no. is less than or equal to 9 or carry is 0 than it is fine but if its sum is greater than 9 or carry is 1 than we must add 6(110) to the output. lets draw a binary table from 1 to 15 (s0,s1,s2,s3) and y as carry.From 1 to 9 carry i.e Y=0 and than from 10 to 15 carry i.e Y=1,

s3	s2	s1	s0	Y
0	0	0	0	0
0	0	0	1	0
0	0	1	0	0
0	0	1	1	0
0	1	0	0	0
0	1	0	1	0
0	1	1	0	0
0	1	1	1	0
1	0	0	0	0
1	0	0	1	0
1	0	1	0	1
1	0	1	1	1
1	1	0	0	1
1	1	0	1	1
1	1	1	0	1
1	1	1	1	1

now the K MAP of this brinary table

s3 s2 s1 s0	00	01	11	10
00	0	0	0	0
01	0	0	0	0
11	1	1	1	1
10	0	0	1	1

on solving we get Y=s3s2+s3s1, let b3b2b1b1 amd a3a2a1a0 are 2 bcd inputs

The output of combinational circuit is connected to b1b2 inputs of adder-2 and b3=b1+0 b3=b1+0 as they are connected to ground permanently. This makes b3b2b1Bb0= 0110 if Y' = 1.

The sum outputs of adder-1 are applied to a3a2a1a0 of adder-2. The output of combinational circuit is to be used as final output carry and the carry output of adder-2 is to be ignored.