[SI-LIST] Re: How to connect Chassis ground to DGND

Steve, 
I have a complete open mind on this one. However, if what you say is
true, there will be a lot of FCAL disk subsystem on fire now since
they are mandatory DC shorted together.

As for the minimal differences between AC short vs. dead short, my
suspicion is the disk drivers and add on card themselves have already
done the dead short for you already. Whether you want to use cap
or zero ohm short on system board does not matter anymore.

-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]
Sent: Friday, February 27, 2004 2:44 PM
To: Chris Cheng; cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: How to connect Chassis ground to DGND


Chris, ground seems to confuse a lot of people.  DC isolation is usually 
motivated by limiting loop currents in shelf to shelf or equipment to 
equipment cables.  It can be important.  Shields in computer room equipment 
cables have been measured carrying many 10's of amperes.  A company that I 
know of decided to "protect" themselves by only using optical connects 
between shelves.  Then they forgot to isolate the RS-232 craft maintenance 
ports.  They managed to set fire in one customer's installation when the 
customer connected one PC to two shelves.

I had another case where due to powering requirements, we needed to DC 
isolate.  Given the opportunity in new equipment that didn't have the same 
powering issues to get rid of the DC isolation, I jumped at the 
chance.  This completely baffled my colleagues who were and remain smart 
people.

I can't explain the random goat sacrifices and other rites that some seem 
to use with add on cards other than the observation that mythology doesn't 
die very easily.  PCs have some issues because the power return and logic 
ground are not isolated.  So, the power return does run through the chassis 
metal.

If your EMI scan showed no difference you are lucky.  Usually the caps 
don't do as well as bonding.  At least that has been my experience.

Steve.

At 01:56 PM 2/27/2004 -0800, Chris Cheng wrote:
>Steve,
>
>This is an interesting area I am always curious. If you look at the GBIC
>spec, it requires DC isolation at the connector point between chassis
>and logic gnd. FCAL on the other hand is exactly opposite and requires
>dead short between chassis and logic gnd. I never quite figure out
>why two huge groups of very smart people can come out with specs that
>are completely opposite to each other. It certainly makes my grounding
>scheme looks funny.
>
>To make things worst, a lot of add on cards like PCI cards seems to
>have a grounding scheme based on the phases of moon. And most of
>the disk drivers vendors like to tie their chassis to logic gnd
>internally just to make your integration a little more complicated.
>
>EMI engineers always told us to DC isolate the entire chassis with
>a single shorting point between chassis and logic ground. The rest
>of the system have decoupling caps to AC connect the chassis to
>logic ground. I never quite understand the logic and I have
>experimented with DC shorting the entire chassis with logic ground
>at every point. The EMI scan shows no difference.
>
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