Hi,
First look at a positive logic AND gate truth table:
(this is arranged as inputA,inputB,outputC)
AND GATE
0 0 0
0 1 0
1 0 0
1 1 1
Now note that we can read this two ways...
Way 1:
A '1' on BOTH inputs produces a '1' on the output, thus we say that
'1' AND '1' equals a '1' so it's an AND gate.
Way 2:
A '0' on EITHER input produces a '0' on the output, thus we say that
'0' OR '0' equals a '0' so it's an OR gate, but because we looked at zeros this
time we say that it is 'negative' logic.
Thus, a positive logic AND gate is a negative logic OR gate.
An example would be the 7400 series gate package, which has four
AND gates (they are called by their positive logic name but can still be
used for negative logic).
Often systems will use gates both ways.