Subject: Re: what's this machine check mean?
To: None <mouse@Rodents.Montreal.QC.CA,>
From: Ross Harvey <>
List: port-alpha
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
> isn't.

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 However, the doctine is obsolete, because there is so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 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.