# Homework Help: Parallel adder SOP Expression

1. Feb 19, 2015

### kukumaluboy

My working:
S3 = A
S2 = B
S1 = C'
S0 = D'

F = Sum-Minterm(0, 1, 2, 4, 5, 8) (Basically i find when is X smaller then Y)
Used Kmap to arrive at
A'C' + B'C'D' + A'B'D'

Am i correct?

2. Feb 19, 2015

### lewando

Did you check your result? If ABCD = 1111, what should F be?

3. Feb 19, 2015

### kukumaluboy

Ok my formula was Si = Xi XOR Yi XOR C

That was wrong. Should be Ci. hmmmm

I need clue bro lol

4. Feb 19, 2015

### lewando

Focus on the relationship between the output F and the inputs ABCD. In other words: treat the whole thing as a black box with inputs ABCD and output F.

5. Feb 19, 2015

### kukumaluboy

So must i like do the excess-3 table and compare ab < cd values

6. Feb 19, 2015

### lewando

"Excess-3", strictly speaking, operates on BCD (binary coded decimal) values. Since the input is not constrained to BCD, it's better to say "So must I like just add 3 to the input..."

Not making fun, just trying to make a fine point. And yes, then do the comparison.

7. Feb 19, 2015

### kukumaluboy

yeayea! thank you man solved it lol. I was thinking it too complicatedly.