Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!ucdavis!sunny!poage
From: poage@sunny.UUCP (Tom Poage)
Newsgroups: comp.dcom.modems
Subject: Re: null modem cables
Summary: You SHOULD use frame ground!
Keywords: prevent nasty offset potentials!
Message-ID: <331@sunny.UUCP>
Date: 9 Sep 88 21:57:32 GMT
References: <8809030943.AA02215@ucbvax.Berkeley.EDU> <2066@stpstn.UUCP> <829@sceard.UUCP>
Reply-To: poage@sunny.UUCP (Tom Poage)
Followup-To: ucbvax!ucdavis!sunny!poage
Organization: UCDMC Clinical Engineering, Sacto., CA
Lines: 32

In article <829@sceard.UUCP> mrm@sceard.UUCP (0040-M.R.Murphy) writes:
...
>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.

The result of miswiring the power to any properly manufactured
computer by reversing hot and neutral will be an increase in 60
Hz leakage to ground through the third wire (chassis ground) by
capacitive coupling.  If per chance hot and CHASSIS ground (the
third prong on your computer or peripheral) were swapped, the
computer would blow a fuse at the switch box before anything
ever got turned on.  For those devices that have only two
prongs, they are normally isolated.  It makes no difference if
the prongs on this type of device are switched.  The risk to
computers and peripheral equipment is relatively low.  In
addition, all surges across pin 1 of, e.g., an RS-232 connection
go directly to ground through the chassis of either device  --
ISOLATED FROM THE REST OF THE DEVICE AND IT'S POWER SUPPLY.

Now for the advantages of pin-1.  Since the lines which carry
signals, including signal ground, are isolated from chassis
ground, a potential problem arises.  Ground wires in the walls
have a finite resistance, and as a result two pieces of
equipment may be operating at different DC potentials from each
other relative to chassis ground.  If this DC offset is great
enough, there is a risk of blowing the circuitry that drives the
signals.  The purpose of pin-1 is to reduce or eliminate this
potentially catastrophic DC potential.  In some cases, pin-1
could cause problems: lightning storms for one; but in this case
you should shut the stuff down anyhow.
Tom Poage UCDMC ucbvax!ucdavis!sunny!poage