Re: [SI-LIST] : Cables with driven shields, was "FCAL DB9 cable shield"

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From: David Instone (david_instone@uk.xyratex.com)
Date: Fri Nov 19 1999 - 02:20:13 PST


See comment inserted 1&3 paras down
Dave Instone
_________________________

Bob Davis wrote:
>
> Josip and List
>
> 10BaseT Ethernet does not have hard ground on each end of the cable and uses
> the transformer for isolation with AC grounding through a cap, which may be
> why it is works so well being wired and serviced by users (a good design
> check list item). FC is capacitor isolated for the electrical signals.

but the shield is NOT isolated.[DI]

>
> Short runs within a chassis/rack rarely get you in trouble, if you can
> guarantee that someone will not put a long cable on it. For the longer runs
> read IEC 950 TNV wiring considerations for Telecommunications Lines (and
> local variations such as UL1950). The problem of power GROUND domains is
> real and has melted real cable ground connections. Mother Earth can
> generate low voltage differences with very large currents between ground
> points at opposite ends of a large building (a hospital addition in this
> case). I understand that the voltage between ground domains is determined by
> soil composition, geology, and current weather conditions. I may have a
> chance to study these in my next lifetime.
>
> Any communications cable that transverses ground domains needs to have all
> lines DC isolated and usually the shield is AC grounded at one or both ends.
> If one end is hard grounded it is usually the transmit end. You cannot
> trade SAFTETY for EMI/EMC concerns.

Exactly, which is why the FC specs say that equipments connected by an
electrically conductive medium must share the same homogenous ground
[DI]

>
> Bob Davis, Consulting Engineer
> Signal Integrity Specialist
> Summit Computer Systems, Inc.
> bob@scsi.com
> (408)353-2706
>
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
> Josip.Popovic@worldheart.com
> Sent: Wednesday, November 17, 1999 9:11 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Cables with driven shields, was "FCAL DB9 cable
> shield"
>
> 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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-- 
Regards

Dave Instone. Compliance Engineer Test Systems, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel +44 (0)1705 443071 (direct line) From 22 Nov 99 use +44 (0)23-92-496862 Fax +44 (0)23-92-499315 http://www.xyratex.com Tel: +44 (0)23-92-486363

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