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

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From: Bob Davis (bob@scsi.com)
Date: Wed Nov 17 1999 - 19:09:56 PST


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.

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.

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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