From: Rachild Chen (firstname.lastname@example.org)
Date: Fri Oct 29 1999 - 02:06:32 PDT
Sorry.I only know 20H,but what's 2W?
Douglas McKean wrote:
> At 08:40 AM 10/28/99 -0500, Mike Mayer wrote:
> >Part of it is that people want "rules of thumb". Alot of these issues
> >are complicated enough that there are few general rules. The other
> >thing is, as you point out, the dependence on empirical data. The
> >CompactPCI Specification says that "CompactPCI boards shall not
> >connect earth ground (on front panel) through a low impedance path to
> >logic ground used on board." I seem to recall seeing a paper measuring
> >the difference in radiated emissions between boards with logic ground
> >connected to chassis ground and with them connected, the conclusion
> >being that emissions were lower when they were connected. The
> >CompactPCI spec does not say why they require them to be isolated, si
> >I don;t know if there is a good reason or if they were just using a
> >"rule of thumb".
> Rules of thumb are part of the tool kit for engineers.
> As long as one is aware from whence they originate,
> I have no problem with anyone using them. It's when
> the person who isn't even partially aware of the origins
> uses them blindly. There have been several debated
> here quite extensively. 2W and 20H are just a couple.
> Traversing across split grounds is another and there
> seem to be pros and cons with that under special
> conditions as well with the general consensus being
> "don't do it."
> The PCI bus being a reflective based system is my own
> personal nightmare. I'm not sure who came up with it,
> but I can only conclude the very last thing concerned
> was EMI. My own experience and talking with people at
> the labs tells they can be a common problem. Usually
> when confronted with a problem at 132, 198, or 330 MHz
> the knee jerk question is "Ya got a PCI bus in there
> doncha?" One of just a couple counter examples where
> one can have excellent SI and terrible EMI. - Doug
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