RE: XAUI AC coupling
Rich,
I would strongly oppose allowing DC-coupling in the spec since we would then
have to pick a DC level. This would certainly hamper some technologies over
others and lead to sub-optimum solutions, either now or in the future.
Pluggable modules need to be AC coupled. Whether or not someone wants to
use a proprietary chip-to-chip solution on a backplane is another issue, one
that should be outside the scope of the standard.
So given your choices below I vote for only AC-coupling.
Regards,
- Richard
> -----Original Message-----
> From: Rich Taborek [mailto:rtaborek@xxxxxxxxxxxxx]
> Sent: Friday, September 22, 2000 10:20 PM
> To: HSSG
> Subject: Re: XAUI AC coupling
>
>
>
> All,
>
> By my count, I have 4 votes for allowing XAUI DC-coupling against 0
> votes for requiring only AC-coupling. The 4 votes are:
>
> Ed Grivna - Cypress
> Dawson Kesling - Intel
> Jeff Porter - Motorola
> Rich Taborek - nSerial
>
> Any other opinions out there?
>
> Best Regards,
> Rich
>
> --
>
> "Jeff Porter (rgbn10)" wrote:
> >
> > Folks,
> >
> > I feel consensus emerging here.
> >
> > Rich writes
> >
> > > a) A XAUI implementer can always get away with AC-coupling and
> > > AC-coupling details for XAUI are readily available;"
> >
> > and
> >
> > > That said, I'd be happy to go with (1) or (2).
> >
> > Dawson writes
> >
> > > An alternative is to mandate CAPABILITY for AC coupling.
> This allows DC
> > > coupling where compatible implementations permit, but
> ensures that ALL
> > > implemenations will interoperate via AC coupling.
> >
> > I agree. Specify the differential signal. Require the receiver
> > to function *when* driven by ac coupled signals to provide a method
> > that insures interoperability. After all, we've increased
> baud rate, among
> > other reasons, to permit ac coupling as an approach to
> interoperability.
> > Do not require ac coupling since dc coupling will often
> work, and we've
> > left a way to interoperate.
> >
> > The remaining technical work is to include in an
> (informative) XAUI link
> > budget (if we choose to explain how this could work) the
> attenuation,
> > skew, and jitter, etc. budgeted for ac coupling.
> >
> > Proposals and justification for this budget item?
> >
> > Jeff
> >
> > Rich Taborek wrote:
> > >
> > > Dawson,
> > >
> > > In terms of specsmanship, I believe that we have two
> alternatives with
> > > regard to coupling for XAUI:
> > >
> > > 1) Leave coupling out altogether as an implementation detail;
> > > 2) Specify detail for both AC-coupling and DC-coupling.
> > >
> > > It sound like you're leaning towards (2) where I'm
> leaning towards (1).
> > > My argument is that (2) is a whole heck of a lot more
> work than (1) and
> > > may be more costly since compliance verification has some
> non zero cost.
> > > I believe that (1) works and is interoperable because:
> > >
> > > a) A XAUI implementer can always get away with AC-coupling and
> > > AC-coupling details for XAUI are readily available;
> > > b) A savvy XAUI implementer may save $$$, increase
> reliability (fewer
> > > components), increase signal fidelity (fewer vias), etc.
> by going with
> > > DC-coupling if possible given their component selection.
> > >
> > > The only other possibilities are not palatable to me:
> > >
> > > 3) Mandate AC-coupling;
> > > 4) Mandate DC-coupling.
> > >
> > > That said, I'd be happy to go with (1) or (2).
> > >
> > > Best Regards,
> > > Rich
> > >
> > > --
> > >
> > > "Kesling, Dawson W" wrote:
> > > >
> > > > Rich and all,
> > > >
> > > > I agree that it would be nice to avoid AC coupling if
> we can still ensure
> > > > interoperability.
> > > >
> > > > If we remove reference to coupling altogether, we must
> add a common mode
> > > > specification or definite logic levels; we cannot only
> specify peak-to-peak
> > > > swing as we are now doing and expect interoperability.
> (All chip-to-chip
> > > > interconnect spec's I know of specify either
> DC-referenced logic levels or
> > > > common mode and differential mode levels. Is there an
> exception? We have
> > > > avoided this by mandating AC coupling up to this time.)
> > > >
> > > > An alternative is to mandate CAPABILITY for AC
> coupling. This allows DC
> > > > coupling where compatible implementations permit, but
> ensures that ALL
> > > > implemenations will interoperate via AC coupling.
> > > >
> > > > -Dawson Kesling
> > > > Intel Corporation, NCD
> > > > 916 855-5000 ext. 1273
> > >
> > > -------------------------------------------------------
> > > Richard Taborek Sr. Phone: 408-845-6102
> > > Chief Technology Officer Cell: 408-832-3957
> > > nSerial Corporation Fax: 408-845-6114
> > > 2500-5 Augustine Dr. mailto:rtaborek@xxxxxxxxxxx
> > > Santa Clara, CA 95054 http://www.nSerial.com
>
> -------------------------------------------------------
> Richard Taborek Sr. Phone: 408-845-6102
> Chief Technology Officer Cell: 408-832-3957
> nSerial Corporation Fax: 408-845-6114
> 2500-5 Augustine Dr. mailto:rtaborek@xxxxxxxxxxx
> Santa Clara, CA 95054 http://www.nSerial.com
>