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Re: XAUI AC coupling

Hi Rich,

I wholeheartedly agree.  AC-coupling should be ALLOWED, but
not REQUIRED.  It is required in 1000Base-CX because the signals
leave the box and are subject to alternate ground domains.

-Ed Grivna

> All,
> I'd like to propose that clause 49, XAUI, remove any requirement for
> coupling, AC or DC. The basis of this proposal is as follows:
> 1) XAUI is a chip-to-chip interconnect. As such, DC-coupling is clearly
> appropriate and advantageous from an implementation perspective, an
> example is when interfacing chips from like logic families utilizing the
> same power supplies. This is likely to be the case in many
> implementations. Therefore, the standard should not dictate AC-coupling
> when DC-coupling is adequate to achieve interoperability.
> 2) I've reviewed all instances of the use "coupling" including fuzzy
> variants in the 802.3 standard. There is no precedent for dictating a
> specific coupling method for a chip-to-chip interconnect in the
> standard.
> 3) Absolutely nothing will be taken away from the standard by removing a
> requirement for AC-coupling. If AC-coupling is either desired when not
> required or required for a specific implementation, then the details for
> AC-coupling including the determination of specific capacitor values,
> the frequency spectrum of 8B/10B transmission code, etc. are all well
> documented and readily available. 8B/10B transmission code is far and
> away the most commonly used and well understood code in serial gigabit
> links including chip-to-chip interconnects. From a signal coupling
> perspective, except for proportionally higher signaling frequency, there
> is no difference between a single XAUI lane and a 1000BASE-X link. Note
> that for 1000BASE-X, both AC and DC coupling is available from
> transceiver module vendors.
> AC coupling was proposed as a requirement for the Hari interface which
> was effectively renamed as XAUI. It has been carried into the baseline
> proposals for P802.3ae. Now is the time to decide whether AC-coupling is
> an interoperability REQUIREMENT. I challenge anyone to argue and prove
> that AC-coupling is required for XAUI interoperability. If such proof is
> not forthcoming, clause 49 should be modified to remove any requirement
> for AC-coupling.
> -- 
> Best Regards,
> Rich
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