A Simple Truth Table

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

1. AARRGGHHH Thread Starter New Member

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

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
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1,227
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
6,255
970
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
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1,355
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.

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
2,424
1,355
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
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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
2,424
1,355
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