PCI 5V signal

[Sek_Shiang TAN <sstan@ecr.mu.oz.au>]

Hi guys,

I am new to PCI Bus. i am just wodering why do we need so many 5V finger
pins on the edge connectors? There are a total of 13 5V pins, especially
at the end corner of the edge connector, 4 5V pins seat next to each
other.  Do i have to connect them all to the 5V power plane?  

I was adviced to put some low speed tantalum capacitor(minimum 10 uF),
located no more than 300 mils from the PCB's power entry points(edge
fingers). This is used for power decoupling.  Do i have to connect the
tantalum cap to each 5V pins. There are 13 of them in total, so the caps
would take up a lot of space. 

Also i need some high speed caps(0.01uF) to decouple any unused 3.3V pins
on the edge connectorto the ground plane. There are 12 3.3v pins there!
So i would need to locate more than 20 capacitors near the edge
connector!! Even if i can bus a few power pins onto one cap, it still take
up a lot of space. I am runnning out of space now.


so i am wondering how do you guys usually tackle this problem?? How many
capacitors(both low and high speed) do you guys used. I have seen one
example PCI card with only a few caps to decouple 3 to 4  3.3V pins, there
are still a few unused 3.3V there without decouple caps. Also they didn't
have any caps for the 5 V signal pins.


Just for your information, i am designing a 5V 32bits PCI add-in card. 

Thanks

Regards

Tan