FW: 3.3V Decoupling

["Taylor, Mike" <mike@buntypost.dundee.ncr.com>]

 ----------
From: 	pci-sig-request[SMTP:pci-sig-request@znyx.com]
Sent: 	21 November 1996 17:56
To: 	Mailing List Recipients
Subject: 	Re: 3.3V Decoupling

part1 (text/plain)
 ------------------------------

> Date:          Thu, 21 Nov 1996 15:50:35 -0500
> From:          Jerry Sabath <sabath@atla3.agfa.com>
> Subject:       3.3V Decoupling
> To:            Mailing List Recipients <pci-sig-request@znyx.com>

> Section 4.4.2.1 of the PCI Spec Revision 2.1 states:
>
>   Additionally, all +3.3V pins (even if they are not actually
>   delivering  power) . . . on the PCI edge connector provide
>   an AC return path, and must have plated edge fingers and be
>   coupled to the ground plane on the add-in board . . . to
>   ensure that they continue to function as efficient AC
>   reference points . . ."
>
> Can someone explain why the specification does not allow
> unplated fingers? Would a board with unplated +3.3V fingers
> have significantly different characteristics than an
> unpopulated PCI slot?

The decoupling of the 3.3V pins is so that they act as an AC ground
for YOUR board...  There aren't enough ground pins on the PCI bus
alone to provide this function...

 -jrp

This assumes that motherboards which do not provide 3v3 to the slots   
follow a similar decoupling scheme!!!
PCI 2.1 covers plugin cards, but does not appear to cover AT style   
motherboards or LPX style and their riser cards. (or have I missed it?).
Mike.
9,