Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site ulysses.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!smb
From: smb@ulysses.UUCP (Steven Bellovin)
Newsgroups: net.dcom
Subject: Re: lightning, PACXs and computers
Message-ID: <1028@ulysses.UUCP>
Date: Fri, 2-Aug-85 16:31:59 EDT
Article-I.D.: ulysses.1028
Posted: Fri Aug  2 16:31:59 1985
Date-Received: Sat, 3-Aug-85 06:06:35 EDT
References: <1397@islenet.UUCP> <29116@lanl.ARPA>
Distribution: net
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 16

> The latest advice (from one of DEC's field people) is that this is a
> typical problem with sites where equipment is in several buildings;
> that, during an electrical storm, each building will have a different
> potential to ground!  Thus, the ground wire in the rs232 cables suddenly
> carries the difference.  I have disconnected (at both ends) all of the
> terminals outside the computing center until we can get protection on
> the data lines.  We have had several lightning storms since, and (knock
> on wood) no more damage.
> 

I'd say that DEC is right.  RS-232 pin 7 ties together the electrical grounds
on both ends; any difference in ground potential -- whether from lightning
or simply a different electrical feed -- can cause trouble.  A solution --
damnifIknow; we tried all sorts of things when I was at UNC Chapel Hill
without notable success.  Optoisolators tend to protect the equipment but
get fried themselves, but they're often better than nothing.