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RE: universal pci card into a pci-x slot

Victor Piron wrote:

> Thanks, but I am looking for current consomations. What is the maximal
> current thay a PCI-X slot can supply?
> I need drink 500 mA per board. Is it possible?
> 
Andy replies:

PCI-X revision 1.0b says, "Device and add-in card power supply voltages
and tolerances and add-in card load limits are the same as for
conventional PCI devices and add-in cards."

Conventional PCI revision 2.3 (and previous versions) give the maximum
currents allowed per slot as:

3.3V:	7.6 A max. (system dependent)
5V:	5 A max. (system dependent)
12V:	500 mA max.
-12V:	100 mA max.

"There are no specific system requirements for current per connector on
the 3.3V and 5V rails; this is system dependent."

The total power consumption allowed on a card from all power rails is 25
watts, or less depending on your PRSNTn# pins.

About that "system dependent" part.  Let's say a system has six PCI
slots.  At 25 watts per card, that's 150 watts.  A given system might be
designed to provide only 100 watts max, so all six cards can't be 25
watt cards in that system.  Other systems might handle six 25 watt cards
just fine.

But wait.  All six cards might draw all 25 watts from the 5V supply.
Six other cards might draw all 25 watts from the 3.3V supply.  Other
cards might draw six watts from +12V and the rest from the other
supplies.

So does the system designer make his system capable of supplying 343
watts to those cards, knowing that at least half that power never gets
used?  More likely, he makes a compromise by using a smaller power
supply, and may further limit the power (current) per supply voltage.
So perhaps his system can provide only 100 watts from 3.3V, 75 watts
from 5V (not at the same time), etc.

What this means, is that certain combinations of power-hungry cards
might not work.  All six cards drawing all of their 25 watts from 3.3V
or from 5V might not work, even though a mixture of other 25 watt cards
might be fine.

Regards,
Andy