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In the AND circuit, the
cathodes are held low by
the "pull down"
resistors.
The diodes are forward
biased and the output is
held low.
If two of the diodes are
taken high to reverse
bias them, the remaining
diode still holds the
output low.
A and B and C must all
be taken high to make
the output go high.
In the OR circuit, both
anodes and cathodes of
the diodes are held low,
and the output is low.
If A or B or C is taken
high, then the
associated diode is
forward biased and the
output goes high.
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