RE: XAUI AC coupling
XAUI is an _optional_ compatability interface. Since it is optional, a chip
to chip or internal to box application is not required to implement it. If
you don't call the interface XAUI, you can implement it any way you want.
Its purpose as a compatability interface will not be served by specifying
options that allow incompatible devices both meeting "XAUI" specs.
XAUI should be specified as AC coupled. Internal DC coupled chip interfaces
would be allowed because they are not required to be XAUI interfaces.
From: Hakimi, Sharam (Sharam) [mailto:hakimi@xxxxxxxxxx]
Sent: Tuesday, September 26, 2000 3:37 PM
Subject: RE: XAUI AC coupling
I also agree that XAUI has chip to chip or internal to a box applications
where AC coupling would not be required and the decision should be left to
the designer. AC-coupling should not be a requirement.
> 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
> > -------------------------------------------------------
> > Richard Taborek Sr. Phone: 408-845-6102
> > Chief Technology Officer Cell: 408-832-3957
> > nSerial Corporation Fax: 408-845-6114
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