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

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From: Chris Cheng (hycheng@3pardata.com)
Date: Wed Nov 17 1999 - 17:48:10 PST


here is the fun part.
i think jim's opinion is based on the assumption that the
noise current comes from the common mode return current
due to imperfect differential FCAL signals in which
case you want the return current to experience as low
impedance as possible and to minimize the loop between
the common mode component of the FCAL pair and the
return current through the shield.
on the other hand ron's opinion is based on the assumption
that local noise current NOT generated by the FCAL pair
will loop thru the shield and thus created EMI problem.
i guess if the common mode component of the shield current
is slow enough speed then both opinions can be met. however,
my suspicion is that the common mode component should
at least contain spikes due to the inbalance edges
which is inheritantly highspeed.
so.... what should we do ??? sigh......
one more thing, dave instone's mail earlier seems to
suggest that opening one end with capacitor isolation is
violating the FC-PH3 spec. besides, classic EMI textbook calls
for opening on the floating end. i think i only know how to
float the receiving end but not the driving end in FCAL i/o
so in a full duplex DB9 cable, i am out of luck.
more sigh ...........
chris

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
 [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Ron Miller
Sent: Wednesday, November 17, 1999 10:08 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : FCAL DB9 cable shield

Jim & Chris
The ferrite bead is needed on the shield to reduce EMI. It will absorb
circulating currents flowing through the shield returning through the
system and rack grounds. Since it is absorptive rather than inductive
it actually terminates this energy. It should be designed to absorb power
at a frequency associated with the edge rates of the signal. And it shoud
be a reasonable short at the data rate. An open at one end will eliminate
low frequency (<1 Mhz) power chopper ground currents. Capacitance can
ensure connection at higher frequencys.
Ron Miller
"Knighten, Jim L" wrote:
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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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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