Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!agate!labrea!polya!kaufman
From: kaufman@polya.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.dcom.modems
Subject: Re: null modem cables
Keywords: sparks loud noises flashes of bright light
Message-ID: <3920@polya.Stanford.EDU>
Date: 10 Sep 88 06:15:28 GMT
References: <8809030943.AA02215@ucbvax.Berkeley.EDU> <2066@stpstn.UUCP> <829@sceard.UUCP>
Reply-To: kaufman@polya.Stanford.EDU (Marc T. Kaufman)
Organization: Stanford University
Lines: 31

In article <829@sceard.UUCP> mrm@sceard.UUCP (0040-M.R.Murphy) writes:
>In article <2066@stpstn.UUCP> aad@stpstn.UUCP (Anthony A. Datri) writes:

..>A full-blown, general-purpose null-modem cable would be thus:

..>1---------1  (always friendly to include the frame ground)

>Including the 1-1 connection can really be exciting if someone (electrician,
>homeowner, facilities group, ...:-) has miswired the AC and reversed
>hot and neutral somewhere. The 1-1 connection can then become a fuse.
>It's not always a good thing to connect frame-frame.

Generally, if this happens (and I HAVE seen it happen), pin 7 (logic ground)
will also be at 115 volts, since most equipment connects logic ground to frame
ground somewhere.  Note that, even if the electrician has miswired the socket,
most likely nothing will happen since in either linear or switching supplies
there is a transformer between the AC and secondary voltage sides.  Where I
found problems was in a (TVI 950) terminal, where the supposed "neutral" side
of the line was connected to the frame through a resistor, to keep the frame
from floating above ground (did you ever rub your fingers lightly on a chassis
and get a slight tingling?  That is from the capacitive coupling between the
hot side of the line and the chassis... sometimes due to deliberately placed
capacitors which are intended to keep RFI out of the device).

The correct use of frame ground in RS-232 environments is to provide
electrostatic shielding for the signal lines.  If you are using Belden cable
with an overall shield, connect pin 1 to the shield at ONE END ONLY of the
cable.  Then make sure that the equipment is correctly grounded.  The connection
at one end only prevents current flow on the shield which can couple into the
signal lines.

Marc Kaufman (kaufman@polya.stanford.edu)