Subject: Re: what's this machine check mean?
To: None <mouse@Rodents.Montreal.QC.CA, email@example.com>
From: Ross Harvey <firstname.lastname@example.org>
Date: 04/17/2000 14:10:01
> From: der Mouse <mouse@Rodents.Montreal.QC.CA>
> > So, while I'm asking, the documentation says that one particular part
> > of the board needs to be connected to the case for ground.
> > Is that true?
> I don't know. Mine isn't - I have no case around the board to connect
> it to, and it's not clear from the doco just what it is that it expects
> to be connected to: power supply output ground, earth ground, what?
> > Is it conventional to have motherboards connected to the case? Isn't
> > this sort of thing best handled through the power supply?
> Very few power supplies have an earth-ground output line. Ideally, all
> of the power supply's output lines are well isolated from any input
> lines (though this is admittedly unlikely to be true of a switching
> power supply); the connection the doc speaks of may be a precaution to
> ensure that the board ground is earthed even if the power supply ground
OK, the story is...
* Tradionally, power supply outputs float, both the + wires and also
the ("ground") - wires. However, anything goes in the PeeCee world
and I notice that the PeeCee PS on my desk seems to have grounded
the black secondary wires.
* The general doctrine says, ground the board to the case at one
place. However, the doctine is obsolete, because there is so
much capacitive coupling to everything nearby at today's high
speeds, it's sometimes better to punt and ground the board at
every opportunity...at least at every mounting hole and possibly
also at every add-in card latch.
* Don't worry about your setup too much .. the board is a world
unto itself, and doesn't really care where ground is until you
start connecting it to other things, which generally supply
a good ground or, like serial I/O, are somewhat tolerant of
bad ones. One reason to ground to the case everywhere is so
the silly single-ended (non-differential) drive standards
everyone uses see a psuedo-ground AC return path in the metal
of the case walls. None of these things usually affects basic
ops, but it may experience transient failures when someone walks
up and touches it.
So, metal standoffs are better, but lots of systems seem to work semi-OK
with all plastic ones.