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Re: XAUI AC coupling


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,

"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       
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