RE: [SI-LIST] : FCAL DB9 cable shield

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From: Chris Cheng (hycheng@3pardata.com)
Date: Tue Nov 16 1999 - 18:01:44 PST


jim,
  excellent point. one follow up question, should the shield/connector
housing be shorted to chasis/local ground through
a) direct short
b) ac short through ferrite beads that have more transfer impedance
then both the shield and the connector ?
chris

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Knighten, Jim L
> Sent: Tuesday, November 16, 1999 5:35 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : FCAL DB9 cable shield
>
>
> Fibre Channel cables are used for relatively short runs (a few tens of
> meters maximum). For short cable runs, ground potential
> differences within
> facilities, hence ground loop currents, are not an issue of concern.
>
> What is an issue of concern is EMI from these cables. In all of our
> applications, EMI is generated by the signal within the cable.
> It is caused
> by differential signal imbalance, which results in a net
> common-mode current
> on the signal pair. This common-mode current returns to its
> source via the
> inner surface of the cable shield. It leaks out of the cable assembly
> through the inherent leakage of the cable shield (i.e., the
> surface transfer
> impedance of the cable shield) and the though the connector. In general,
> the connector is the biggest emitter of EMI. In the case of a properly
> shielded connector (conducting backshell and a 360 degree bond to
> the cable
> shield) then the leakage mechanism is the connector shield's transfer
> impedance. This is usually bigger than the cable shield's transfer
> impedance, but is manageable. In the case of a cable shield that is not
> connected to the connector shield, then the leakage of common-mode EMI
> radiation is very high at the connector. That's why these Fibre Channel
> cable assemblies have continuous shields from cable to connector and then
> onto to the chassis wall that contains the mating connector.
>
> As far as Ethernet is concerned, the specifications prohibit grounding the
> shield of the cable to the chassis through the connector (except
> at a single
> location). This is a prime reason why Ethernet is such a horrible EMI
> offender. It presents a highway for noise internal to a chassis
> to ride out
> on the ungrounded cable shield.
>
> Jim
> ________________________________________________________
> Dr. Jim Knighten e-mail: Jim.Knighten@SanDiegoCA.NCR.com
> <mailto:Jim.Knighten@SanDiego.NCR.com>
> Senior Consulting Engineer
> NCR
> 17095 Via del Campo
> San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com>
> Tel: 858-485-2537
> Fax: 858-485-3788
>
> ***** Notice the Area Code change from 619 *****
>
>
> -----Original Message-----
> From: Christian S. Rode [mailto:csrode@mediaone.net]
> Sent: Tuesday, November 16, 1999 4:57 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : FCAL DB9 cable shield
>
> Certainly any difference in ground potential between
> locations could
> cause
> current to flow preferentially through the shield. I don't
> think being
> on
> different phases is as much the issue as unbalanced use. If
> one phase
> of
> your home triplex service is used more heavily or a large
> motor blows a
> phase on three-phase industrial service I think the
> center-ground taps
> will create a voltage differential.
>
> Capacitively or inductively coupling shields and ground at
> one end
> solves the ground current problem but you'll have to choose
> a
> coding scheme without a DC component...
>
> Isn't this why optocouplers were invented?
> Aren't there safety issues, too? Seems I remember that it's
> not to
> (current?) code to run Ethernet cabling between buildings.
> Not that
> people don't do it anyway...
>
> > >
> > Wouldn't this cause nasty galvanic currents through the
> shield if the two
> > pieces of equipment, attached by the cable with the DB9s,
> were, say, on two
> > different phases of a three phase power feed to a
> building.
> >
> > Am I missing something?
> >
> > thanks
> >
> > Tom Gandy
> > Industrial Catalyst
> >
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