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Epo
10-05-2005, 12:33 PM
Hey guys,

During an assignment for my Digital Electronics class, I started trying to simplify a few expressions...well...let me write it out for you.

Given inputs: A, B, C, D
Where, b = (not)B

Here is the starting equation:
AB + AbCD + CD

Now, basic rules state that I can eliminate the AbCD. Which makes sense, because:
If CD is 0, then AbCD is 0
If CD is 1, then AbCD doesn't matter

So the initial equation simplifies to:
AB + CD

Then I started thinking...

Again, back to the original equation:
AB + AbCD + CD

If B is 0, AB is eliminated, and AbCD relies solely on ACD
if B is 1, AB relies solely on A, and AbCD is eliminated

Therefore, does that not simplify to:
A + ACD + CD
Which further simplifies to:
A + CD (Same thought process as the first outcome)

However, these two conclusions imply that:
AB + CD = A + CD

Which just makes my mind want to implode. Did I do an error in my thought process? Does B ACtually not matter? Are they actually equal?

This assignment's already been submitted, and I stuck with the first simplification (including the B). Can anyone tell me where I went wrong in that second simplification though?

I would just like to know if I'm rationalizing everything incorrectly, or else this may come back to bite me on an exam (and the prof isn't what you could really call easily accessible). I posted it here because I figure most college/university programmers have gone through an introductory Digital Electronics course.

If anybody can shed some light, it would be much appreciated.

Thanks.

PJYelton
10-05-2005, 01:00 PM
I don't have time to look at your calculations, but there has to be an error in this reasoning:
If B is 0, AB is eliminated, and AbCD relies solely on ACD
if B is 1, AB relies solely on A, and AbCD is eliminatedsince for A=1, B=0, C=0, D=0 the statement AB + AbCD + CD is false yet the statement A + ACD + CD is true.

PJYelton
10-05-2005, 01:31 PM
I think your error in logic comes from the thought process of "if B is false then the equation relies on XXX, if B is true then the equation relies on YYY, therefore B must be irrelevant and can be taken out".

Take this simple example: AB + bC. If B is true then the equation relies solely on A, if B is false then the equation relies solely on C. But that does NOT mean the equation simplifies to A + C since you need B to tell you which variable the equation needs to rely on.

pianorain
10-05-2005, 02:00 PM
It's simple to check. Consider the inputs A=1, B=0, C=0, and D=0. A + CD = 1, while AB + AbCD + CD = 0.

Epo
10-05-2005, 04:10 PM
Okay, thanks for the help guys. I was having trouble seeing the difference between the two cases.

But I see now, with the actual rule, I am completely eliminating the term in both cases.

However, in my mixed up logic, I'm only leaving what is relied upon, even if both aren't getting eliminated (hard to put into words)....Another try, I'm not using the whole term, and then, as mentioned (and proven with the examples), "you need B to tell you which variable the equation needs to rely on."

I believe I'm seeing my mistakes now. Thanks for the help guys.