[Soekris] Y adaptor for PCI interface

[Bernd Walter]

On Wed, Mar 14, 2007 at 02:46:26PM +0100, Raphaël Jacquot wrote:
> Andrea Cristofanini -- [GedamEurope] a écrit :
> > Dear all
> > i'm looking forward to find out a Y PCI adaptor for Soekrsi net4801
> > 
> > So exactly i'm looking for an adaptor that let me plug into soekris  
> > more that one PCI interface.
> > Does this exist ?
> > Pls anyone have with info can drop me an email to andrea at gedameurope.com
> > 
> > Besta Regards Andrea
> > Gedam  Group.
> 
> I think you can't do that.

There are many aspects why this would not work.

Ever PCI slot has a unique line selecting this slot.
This line is physically attached to any of the AD* line accoring
to the slot number.
A splitter card can't know which of them is already in use by onboard
components or other slot headers - normaly they ship with a small card
stealing the signal (together with INT*) from another slot header.
Theoretically you can steal lines from the Mini-PCI.

Interrupts are the next aspect - the BIOS can't tell the OS about
how interrupt lines are connected to PCI slot numbers it doesn't know
about.

Another aspect is line length.
PCI is very strict about line lenght and extending the lines is always
risky.
Obviously violating PCI specs in this area isn't that critical, as this
point is although true for other riser-cards contructs that the
motherboard vendor don't support with their board layout.

clock line is the next problem point.
Each slot has a different clock line, since they can be loaded with
a single chip only.
This is typically a limited resource.
Normally the clock comes from the host side of a given PCI bus and
you use an internal clock drivera with just a hand full output lines.
You usually can use an external clock driver with more output lines,
but all components on a given PCI bus must use their clock signal from
the same source.
Unless there are free clock lines you have to rewire onboard components
as well, or violate PCI specs by attaching multiple chips to the same
line - as all cheap riser cards do, so it isn't that problematic in
praxis.

All in all a cheap riser may work given some software hacks, but it
will very likely not work out of the box.

A real working splitter/riser card would have to use a PCI-PCI bridge.
This way you create a new PCI bus with new addressing, new clock,
a well specified interrupt routing, etc...
However those cards are rare.

-- 
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