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

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From: Knighten, Jim L (JK100005@exchange.SanDiegoCA.NCR.COM)
Date: Tue Nov 16 1999 - 17:34:32 PST


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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