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

Chris,
Those FCAL disk systems might not have isolated power
supplies.
I think Steve made that point...
/dan

--- Chris Cheng <Chris.Cheng@xxxxxxxxxxxx> wrote:
> 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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