UPS/SPS testing re: AC protective ground (and RS-232)

[Floyd Davidson]

In article <35425 at cup.portal.com> thad at cup.portal.com (Thad P Floryan) writes:

>
>adams at ucunix.san.uc.EDU (James Warner Adams)
>in <9010282317.AA08682 at ucunix.san.uc.edu> writes in response to my comments
>about RS-232 pin 1 grounding:
>
>	``Pin 7 should NEVER be directly connected to pin 1.  This is a hallmark
>	of poor design.  ... ''
>
>I've never felt "comfortable" about that practice either, but I took my cue
>from AT&T documents.  Been selling from 10 to 50 of (my product) every month
 [text deleted]
>other shielding.  And pin 1 is internally connected to pin 7 on that product.
>You figure.  Quien sabe?  :-)

The point is don't connect them together at the cable connectors.  
Eventually they will come together in one way or another, but never
at the cable. 


>
>	``[... other good material deleted here ...]
>
>	Anecdotal evidence aside, good practice dictates that
>	a single ground reference point be used when cabling.''
>
>VERY TRUE!  I learned that early in the game while designing low-noise
>microwave pre-amps for "government" applications.

That is where they come together.  It might in fact be a site ground
bus located across the room.

>
>	``When installing shielded cables, the shield is generally connected
>	BOTH to the hood AND to pin 1.  There should not be an independent pin
>	1 wire running inside the shield.  This defeats the whole purpose of
>	the shield and invites interference.''
>
>I'll take your word on that (above), but it was my impression a "shield" is an
>electrostatic barrier whereas the "pin 1 ground" is simply a protective drain.

I won't (above).  The protective ground, pin one should be connected
straight through on a single wire, inside the shield.  The shield
should be connected to the hood and pin 1 on one end only.  The other
end should be isolated.  The protective ground is just that.  And
the shield should be just a shield too.  Mix the two and the shield
probably won't.

>	``Thad, did it ever occur to you that perhaps your "site" is one REASON
>	for the awful interference you mention?''
>
>Yep, and the reason for my noisy site (8 computers, 6 modems, and misc. other

I bet that occured real fast! :-) :-)

[ text deleted ]

>Fortunately, the site is located in a sparsely settled area and I've verified
>non-interference with neighbors' TVs, radios, pagers, cellular phones, and

My site is in a fringe area, also "sparsely settled".  I've had to pay
*very* close attention to even where my TV antenna is located relative
to my computer room.  I don't aim the antenna for max signal, I aim for
a sharp null toward all the EMI from the opposite corner of the house!

I've never seen a published in depth technical discussion on the theory
and practice of shielding RS232 cables, but I would suggest that
reading any of the many that are available on audio.  In particular the
grounding of the shield at both ends with a relatively high impedance
line is asking for a ground loop.  That may or may not be bad enough to
interfere with any given RS232 signal.  But if you want to see if it does
happen or not, try it with a high impedance input on your hifi set!

Floyd


-- 
Floyd L. Davidson   floyd at hayes.ims.alaska.edu    floydd at chinet.chi.il.us
Salcha, AK 99714    connected by paycheck to Alascom, Inc.
When *I* speak for them, one of us will be *out* of business in a hurry.