[Beowulf] Re: Emergency Power Off

[Tom Mitchell]

On Tue, Mar 20, 2007 at 02:07:43PM -0700, Jim Lux wrote:
> Date: Tue, 20 Mar 2007 14:07:43 -0700
> To: "David Mathog" <mathog at caltech.edu>, beowulf at beowulf.org
> From: Jim Lux <James.P.Lux at jpl.nasa.gov>
> Subject: Re: [Beowulf] Re: Emergency Power Off 
> Cc: 
> 
> At 01:25 PM 3/20/2007, David Mathog wrote:
> >> At 04:20 PM 3/19/2007, David Mathog wrote:
> >
> >On further consideration, that common ground might be
> >a problem.  On the one hand it's effectively running from one
> >serial port to another and generally that's ok.  On the other
> >hand, if for whatever reason one or more of the grounds is
> >a volt or two higher than another one, that could result
> >in quite a current running on that signal wire.  This would
> >probably be safer:
> 
> 
> or use diodes.  e.g. 1n5711 or 1N5401
> 
> Given that there is probably an alternate grounding path, you 
> probably want two diodes for each UPS (one in each line).
> 
> or, get a switch with 5 sets of contacts and use 5 sets of batteries 
> (I assume you were using Alkaline 9V batteries?)

While thinking about grounding look at it with care
from the UPS event point of view too.

With multiple UPS devices earthing both from  a safety point
of view and a logic (including RS-232/488) point of view should be 
double checked.  Some UPS boxes do very funky things to ground....

I once worked in a building with multiple phase service to
different parts of the building.  The lab was a good distance
from the main panel and both safety ground and neutral could
wander with very large currents if different rooms/walls were
interconnected.  We discovered this when a 24AGW terminal wire
(ground) fuse linked with a flash and the 1488/1489 RS232
devices cratered their packages during an electrical storm.
BTW: All was 100% to code.....

The damage to the computers was from the terminal links.  

Anyhow, check all the wires and what happens to everything when
the events the UPS devices are to protect you from happen.
Think of each UPS as a special AC service drop.



-- 
	T o m  M i t c h e l l 
	Found me a new place to hang my hat :-)
	Now it got bought.