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From: jeq@laidbak.UUCP (Jonathan E. Quist)
Newsgroups: net.dcom
Subject: Re: lightning, PACXs and computers (followup)
Message-ID: <177@laidbak.UUCP>
Date: Fri, 16-Aug-85 00:49:06 EDT
Article-I.D.: laidbak.177
Posted: Fri Aug 16 00:49:06 1985
Date-Received: Sun, 18-Aug-85 03:28:22 EDT
References: <1397@islenet.UUCP> <29116@lanl.ARPA> <1028@ulysses.UUCP> <1831@ecsvax.UUCP> <1465@islenet.UUCP> <1242@umcp-cs.UUCP>
Reply-To: jeq@laidbak.UUCP (Jonathan E. Quist)
Organization: LAI Chicago
Lines: 32
Summary: 

In article <1242@umcp-cs.UUCP> chris@umcp-cs.UUCP (Chris Torek) writes:
>>... All Computer equipment and auxiliary racks within a room should
>>be securely grounded to a single point with generous-size braided
>>ground straps.  It seems to be a good idea to tie down all incoming
>>terminal grounds (RS232 pin 7) to that same point. . . .
>
>Everything I thought I knew about grounding says you never tie signal
>ground (pin 7) to frame ground.  Am I wrong, or is this a Big Mistake?
>-- 

According to the standard (Yes, it is a published standard, though
I haven't actually seen a copy in years), pin 7 is signal ground,
pin 1 is "safety" ground.  While some equipment ties the two together
internally, most _well designed_ distribution panels seperate
the two.  In my experience, several terminals (on long cable runs)
which had noise problems improved when the I cut the strapping
between pins 1 and 7 at the terminal end.  (A colleague and I
had routinely tied the two together until we noticed this.)

If you are running shielded cable (what a concept!) to
your terminals, it may help to tie the shield to ground
at _only_ one end of the cable run.  Since the distribution
panel is likely the only sure ground in the system, all shields
should be tied at that point.

(If I'm beginning to confuse audio practices with digital practices,
light your torches and let me know!)

Jonathan E. Quist
Lachman Associates, Inc.
ihnp4!laidbak!jeq
``I deny this is a disclaimer.''