XOR, XNOR logic, question.

[firewalker]

When do we say that a digital circuit verifies the "XOR logic" or the "XNOR logic". Does a digital circuit with function

Y=A+B+AB

verify any of the above logic?

http://en.wikipedia.org/wiki/Exclusive_or
http://en.wikipedia.org/wiki/Logical_equality

Alexander.

[Dave]

Y = A + B + AB is just an ordinary OR function. The AB part of the equation is redundant.

XOR: Y = AB' + A'B (the ' symbols are the negations)
XNOR: Y = AB + A'B'

[firewalker]

So the question is (once again) wrong. You must select XOR logic or XNOR logic.

Alexander.

[DJohn]

"Verify" is a strange word to use.  A circuit can realise some logic function (make it real), and you can verify that it does.  But what they mean by the circuit verifying the function, I don't know.

If someone claims that Y = A + B + AB is equivalent to XOR or XNOR, it's easy to check with a little truth table:
A B | Y XOR XNOR
0 0 | 0 0      1
0 1 | 1 1      0
1 0 | 1 1      0
1 1 | 1 0      1

The Y column isn't the same as either XOR or XNOR, so it's something different.  As Dave says, it's equivalent to OR: Y = A + B.

But that might not be what the original question is asking.  Do you have a bit more context?

[firewalker]

An exact translation (from Greek) of the question is:

Specify the logic equation of the output of the following circuit. Verifying the EXOR logic or the EXNOR logic? justify you answer.

The syntax is also wrong in Greek. The question "Verifying the EXOR logic or the EXNOR logic?" makes more scene as "Does it verify the EXOR logic or the EXNOR logic?".

And also, who writes XOR as EXOR and XNOR as EXNOR?

Alexander.

[DJohn]

That makes a bit more sense.  This circuit is !(!(A+B))(!AB)) = A+B + AB, as you said.  I guess they're asking if it's XOR or XNOR, but it's neither.

If you turn the NOR gate into an OR, then you get !((A+B)(!AB)) = !(A+B) + AB, which is an XNOR.  Perhaps you should offer to proof-read for them.  For a suitably high fee, of course, to compensate for the inevitable loss of sanity.

I've not seen EXOR before, but the 6502 did have an EOR instruction (for 'exclusive or').