A Simple Truth Table

Discussion in 'Homework Help' started by AARRGGHHH, Feb 20, 2013.

  1. AARRGGHHH

    Thread Starter New Member

    Feb 20, 2013
    6
    0
    A simple truth table that I somehow screwed up:

    EXNOR-GATE.PNG

    I showed the truth tabel for this as follows, with x being my answers:

    A B X
    0 0 0
    0 1 1
    1 0 1
    1 1 0

    That looks sooooo right to me. Can someone explain where I went wrong?

    Thank you
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Yes I see what is wrong. On the output you see a small circle. That small circle is a symbol denoting that the output is inverted. So your circuit is XOR with inverted output. You may also se this on a input. The meaning is the same here
     
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  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    T06afre already mentioned the point.
    You can see the truth table of 74LS86 and compare with yours.
     
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    Exactly. Your truth table is for an XOR gate. The one in the image is XNOR. Simply invert the outputs, as denoted by the small "bubble" on the output, and you're all set.

    Regards,
    Matt
     
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  5. AARRGGHHH

    Thread Starter New Member

    Feb 20, 2013
    6
    0
    Thank you for the replies.

    Okay, I missed seeing that little circle, all I saw was the XOR gate. If I understand correctly, that little circle is basically a not gate, it inverts the output values.

    So, instead of

    A B X

    0 0 0
    0 1 1
    1 0 1
    1 1 0

    The truth table should have been

    A B X
    0 0 1
    0 1 0
    1 0 0
    1 1 1

    Correct?
     
  6. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    1,328
    Exactly right. And yes, the bubble serves the same purpose as a not gate connected to the output or input. Same idea.

    You seem to have it now :)

    Regards
     
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  7. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    And that is why you got the other answer wrong also.
     
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  8. AARRGGHHH

    Thread Starter New Member

    Feb 20, 2013
    6
    0
    Yes, I do, thank you for your assistance. :)

    Yes, I saw that right away as soon as I got this answer resolved. I still do have one outstanding question on the other post, would you mind taking a look? http://forum.allaboutcircuits.com/showthread.php?t=81290 Thanks again!
     
  9. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    It's what we're here for ;)
     
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