AW: 2 layers or more

[Bernd Riemann <BRiemann@Pinnaclesys.com>]

A 2-layer PCI PCB can be done, and I personally did not discover one
combination of motherboard and 2-layer designs I have seen that did not
work, although this card will not be PCI compliant. Keeping within all the
length and geometry requirements will lead you either into violating the
impedance rules, or making the board unroutable.

-----Ursprüngliche Nachricht-----
Von: fmnemeth@rockwellcollins.com [mailto:fmnemeth@rockwellcollins.com]
Gesendet: Mittwoch, 27. März 2002 16:34
An: pci-sig@znyx.com
Betreff: Re: 2 layers or more



I believe that a 2-layer PCI PCB can be done, if the proper care is taken
in the proper places. I have done 2-layer SCSI cards before, & they can
work just fine. I definitely agree with someone else's comment (i forget
who...) who recommended Dr. Howard Johnson's works.

First, use as much decoupling as possible; at least as much as you would
use with a multilayer card, & maybe more. With a 2-layer card, you don't
have this big ground plane to nail your signals to.

Second, keep within all the length & geometry requirements for the PCI
signals; they still apply.

Third, pay careful attention to your signal return paths. All of the PCI
signals (and others, too) need a good signal return path - the best return
path is one that follows the actual signal path with a constant impedance.
This is built-in with in multilayer board (i.e. the ground plane), but you
have to intentionally make it for the 2-layer case. If you run a ground
trace all the way along next to each signal trace, or even every 2 signal
traces, from IC to connector, you will have established signal return
paths. Where you can, maximize the amount of copper that is ground.

I think these are the basics. Dr Johnson goes into more detail on this, but
this is a small start.

mike nemeth
Rockwell Collins, Inc.

---- opinions expressed herein are solely those of the author and not of
Rockwell Collins, Inc. ----



 

                      Iñigo Alvarez

                      Ximenez de Embun         To:      "PCI-SIG, Mail List"
<pci-sig@znyx.com>                                    
                      <patxipatxi@jazz         cc:

                      free.com>                Subject: 2 layers or more

 

                      03/26/02 06:48

                      PM

 

 






Perhaps I'm too ambitious in my "design".
Really I'm an engineering student that wants to assembly his  final project
of career.
I suppose that my prototype card (assembled in a 2 layers  PCB due to my
limited resources) could not fulfill all the PCI specifications  and that I
only must fulfill them in my final design (developed with a more  than 2
layers PCB).
In any case, I suppose that locating a decoupling capacitor in  each 3.3V
and Vi/o pin I will make sure not damaging the PC in that I must prove  the
board, isn't it?
In that case, what could be the maximum trace length  from the PCI card
edge connector to the capacitor?
I also would like to know if the decoupling spec. are valid  for a 4 layers
PCB or if, even in this case, I must use more than one capacitor  for all
3.3V and Vi/o pins.
in another order of things, I have seen an audio PCI board  that it did not
have any capacitor to less than 0.5 inches of distance from the  PCI card
edge connector. Do this means that the board uses all power pins?  If it is
not thus, which could be the explanation (assuming that the board  fulfill
the PCI specifications)?
Thanks


Iñigo Alvarez Ximenez de Embun
e-mail: patxipatxi@jazzfree.com