[SI-LIST] Re: How to connect Chassis ground to DGND
- From: Dan Bostan <dbostan@xxxxxxxxx>
- To: Chris.Cheng@xxxxxxxxxxxx, 'steve weir' <weirsp@xxxxxxxxxx>,cchalmers@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 27 Feb 2004 15:21:00 -0800 (PST)
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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