RE: [SI-LIST] : Cables with driven shields, was "FCAL DB9 cable s hield"

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From: Josip.Popovic@worldheart.com
Date: Wed Nov 17 1999 - 09:10:48 PST


Direct connection from a shield to a connector chassis is the best except if
safety issues are of a concern. Sometimes a capacitor is used to provide an
ac "short" and open dc connection.

Anyone having some experience with cables with driven shields?

Josip

-----Original Message-----
From: Knighten, Jim L [mailto:JK100005@exchange.SanDiegoCA.NCR.COM]
Sent: Wednesday, November 17, 1999 11:24 AM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : FCAL DB9 cable shield

Chris,

In my experience we have provided a direct short from the shield of the
connector to the chassis. This works very well and provides a good
continuous shield envelope around the differential pair and their associated
transceivers.

Transfer impedance is a measure of outer current to inner voltage, so the
lower the transfer impedance is, the better the shield is. So, I'd suspect
that the ferrite bead/ac short approach is not as good.

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: Chris Cheng [mailto:hycheng@3pardata.com]
                Sent: Tuesday, November 16, 1999 6:02 PM
                To: si-list@silab.eng.sun.com
                Subject: RE: [SI-LIST] : FCAL DB9 cable shield

                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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