Re: 3.3V Power on PCI 2.1 motherboards

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

When PCI was implemented, several years ago, most of the PC world was 5V
only.  The preliminary PCI spec would have required 3.3V power at all
PCI connectors.  Many vendors reacted negatively to adding a regulated
3.3V supply just for PCI.  Rumor had it that some PC vendors threatened
to ignore that requirement.  The result was the compromise that made the
3.3V supply "optional" on motherboards.

However, PCI 2.1 also essentially required PCI expansion cards to make
their own 3.3V power, rather than taking it from the motherboard.  So
the incentive for adding (or connecting) 3.3V to the PCI connectors on
the motherboard was essentially negated.

I doubt that many PC vendors actually provided an upgrade path, i.e., a
kit that you could purchase and install.

However, even if they don't stock such a kit, it is essential that they
do "decouple all 3.3V power pins."  The 3.3V power pins are a
significant part of the AC signal return path through the connector, and
even if they don't carry DC power, they do carry transients.  Not
decoupling those pins can significantly degrade the signal integrity of
the bus.  I would call such systems non-compliant.

PCI works on self certification of compliance.  There is a Compliance
Checklist that helps the vendor check that his products comply with the
specs.  And there are occasional Workshops that can help companies test
their products for compliance.  But as far as I know, there is no
governing agency, similar to the USA's FCC, that vendors submit their
equipment to and in return get a certificate saying that they passed.
PCI was designed largely for the mainstream personal computer industry
where "anyone can play."

Andy