RE: Expansion board decoupling specs

["Ingraham, Andrew" <Andrew.Ingraham@compaq.com>]

>Personally I would follow common sense and ignore this part of the spec.
>What if the slot is empty? Will the (unused) power lines be more
>decoupled?

That would be an unwise choice.

They are there for a reason.  The spec says they must be bypassed even if
they are "unused" or "not actually delivering power".

The capacitors aren't there to bypass the power supplies.  They are there to
provide continuity for AC switching currents, through the connector to and
from your expansion board.

When a signal switches, there is a current pulse that follows the signal
wherever it goes.  Electrical currents always take a complete path, from
source to destination and back to the source again.  The laws of physics say
that the switching current going out and the current coming back want to
stay physically close to one another.

In your boards, both motherboards and plug-in cards, the return current is
in the ground and/or power planes, right under the signal trace.

In the connector, the return current has to go through the connector pins
that are near the signal pin.  If you look at the PCI connector pinout,
you'll see that nearly every single signal pin is adjacent to a ground pin
or a power pin.  That is on purpose, to provide a path for each return
current.

Ideally, these would all be ground pins, and then we would have to find some
other way to get power up onto your PCI cards.  But that would be wasteful,
so the PCI connector pinout intersperses power with ground pins.  This works
very well, as long as the power pins are bypassed to ground on BOTH sides of
the connector.

If you eliminate those capacitors on EITHER side of any connector (that the
signal has to actually pass through, i.e., not including empty slots), then
you screw up the return path for the switching current.  Your signal
integrity suffers.  Don't do it.

If you leave them off your PCI expansion card, customers might find that
their PCs crash a lot (or don't boot at all) when they plug your card into
their PC.  That wouldn't look good.

It would be like leaving off all but one of the ground pins on your edge
connector.  Very bad idea.