PCI Load Question

[Todd Harrington <THarrington@TRILOGIC.com>]

I was hoping someone could help clear up a question we have here
regarding PCI loads.

As I understand it each PCI bus can have a maximum of 10 loads. We ofen
see PCI "passive backplanes" with say 3 or 4 PCI slots which is 8 loads
(one for each slot and one for each device that plugs into the slot).
Then you have the CPU connector (The PICMIG conenctor consisting of a
PCI card edge connector and ISA connector) and sometimes there may be
(2) of these, so another 2 loads (one for each connector). Then the CPU
that plugs into the PICMIG CPU connector might have onboard PCI SCSI, a
Host/PCI bridge, and PCI/ISA bridge - 3 more loads. 

As you can see a passive backplane with 2 PICMIG CPU connectors and (4)
PCI slots could take up to 13 PCI loads.

On backplanes like the one I described, or even a backplane with one CPU
PICMIG slot and (3) PCI slots, where there is no PCI-PCI bridge, how do
these backplanes function? It seems like there a lot of systems out
there with (4) slots on it with no bridge. Are systems with more than 10
PCI loads out of spec? Will they work? It would seem any system with a
CPU connector and (4) additional PCI slots would be out of spec. I need
some clarification on what happens when one has more than 10 PCI loads
in a system, for example 11, 12, or 13 loads.

Any help on this subject would be appreciated.

Regards,

Todd Harrington
TRILOGIC
Senior Systems Engineer
Tel -  978-658-3800 x-151
Fax - 978-657-5927
todd@trilogic.com