1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Double negation in logic circuit

  1. Apr 7, 2009 #1
    Probably a very simple answer here...

    I'm creating a logic circuit using only NAND gates. I have 4 inputs, A B C and D, but I am just using a single 'path'/part of the equation for this question, that being A'B = F, F being the output.

    Full equation...

    AB'C + AB'D' + A'B + A'C = F
    F has a NAND gate directly before it, negating any 'paths' leading into F
    I'm just using this highlighted part of the equation as an example


    I have two ideas, I just want to know if one is more appropriate or correct than the other.

    Firstly, split A and feed through NAND gate, giving A'
    Run A' and B through NAND gate to combine into A"B'
    PROBLEM: The input of that path to F needs to be A'B, so A"B' will need to be split and passed through a NAND gate to give the correct input into the final gate, so...
    Split A"B' and pass through NAND gate, giving A"'B" (DeMorgans cancels to A'B - correct?)
    By my thinking, that should work, but it has a lot of split>NAND gate situations in there.

    Split B and feed through NAND gate, giving B'
    Run A and B' through NAND gate, giving A'B"
    PROBLEM: This is where I get muddled...
    Because there's a single negation on A and a double on B, my mind is saying that I can cancel off the one's on B, leaving only A negated, giving the A'B I need BUT...
    Canceling that double negation over B would be considered 'breaking the bar' in DeMorgans, and require me to change the sign between A and B, giving a completely different equation (say A' + B, right?).

    So I have a very strong feeling that a) is the option to go for. Is my thinking right?
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted

Similar Threads for Double negation logic
Mesh analysis -- Where does the negative sign come from?