Karnaugh Map Confusion

Discussion in 'Homework Help' started by Jake1234, Mar 6, 2008.

  1. Jake1234

    Thread Starter Member

    Oct 14, 2007
    19
    0
    I have a question regarding a 3 variable K Map I'm doing in a Schaum's Outline. I 'm solving it using product-of-sums method. I was given the following truth table.

    -------- |----|-------- |-----|
    A B C | Y |A B C | Y |
    -------- |----|-------- |-----|
    0 0 0 | 1 | 1 0 0 | 1 |
    0 0 1 | 0 | 1 0 1 | 0 |
    0 1 0 | 1 | 1 1 0 | 0 |
    0 1 1 | 1 | 1 1 1 | 0 |

    (Sorry for the horrible ASCII animation)

    I derived the maxterm Boolean expression by looking at the 0 output and inverting all the inputs to get (A + B + /C) * (/A + B + /C) * (/A + /B + C) * (/A + /B + /C) = Y and according to the outline this is correct.

    Now I proceeded to draw the K map as follows:


    | C | /C
    ----------------------
    A + B | | 1
    /A + /B | 1 | 1
    /A + B | | 1
    A + /B | |

    Now I circled the two 1's in the /A + /B row, the two 1's in the A + B and /A + /B rows in the /C column and the 1's in the /A + /B and /A + B rows in the /C column.

    Now I know the solution, however I wasn't able to compute it because of the order of the A + B terms. I was very confused as the answer is (/A + /B) * (B + /C) = Y but I kept getting (/A + /B) * /C * (/A + /C) = Y?

    I was just wondering if the order of the A + B terms matters? Because I can switch the order to

    | C | /C
    ----------------------
    A + B | | 1
    A + /B | |
    /A + /B | 1 | 1
    /A + B | | 1

    and get a different map! and the solution to this map is (/A + /B) * (B + /C) = Y!

    I'm probably just confusing myself or thinking about it to much, and this really isn't homework, but it's sort of homework related. I guess I just don't understand why switching the order of the A + B terms gives you a different map with a different answer. If somebody can clarify, that would be great because it's been bothering me for a while.
     
  2. Jake1234

    Thread Starter Member

    Oct 14, 2007
    19
    0
    Ahhh...

    I forgot to hit the preview before posting, now the information is incomprehensible. I'll try to attach the truth table and the maps I did. Sorry!
     
  3. Jake1234

    Thread Starter Member

    Oct 14, 2007
    19
    0
    Yeah I figured it out.

    You had to use the Gray Code scheme to get the K-map right.

    I guess I should of read this, http://www.allaboutcircuits.com/vol_4/chpt_8/6.html

    Sorry....:(
     
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