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Capacitors and VIO clamp



Hello All,

Several people have responded to my post about the need for capacitors on
power pins not used by a PCI plug in board. Apparently, these are indeed
required for the spec, but they seem to be a common missing offense.

People have also commented about the need to implement the VIO pins for
clamp diodes. While this makes sense, I have seen two example designs for
Universal Boards that do not have these clamps. Both of these designs are
from reputable PCI Chip manufacturers. In addition, neither of these
manufacturers uses the VIO pins in their design to connect to the PCI
interface asic. It is like they count on the 5V tolerance of the part as
license.

That is the basis of my confusion -- something that looks like it makes
sense, is not being used by people who I figured should know this stuff.

As time runs, and I look at other people's PCI boards it is becoming more
and more amazing that there are not more system failures out there. I saw
one design where the clock trace was routed directly to the PCI chip (not
snaked) and was only around 1.25 inches. This board also left the Presence
pins completely unconnected. You might be surprised if I told you who's
board this was.

William Benner