RE: XAUI AC coupling
Let's see if we can refine the question, in the hope of making progress.
We do have some common ground: If a PHY module is going to be purchased
by a switch manufacturer, it will likely end up as a pluggable or
solderable module, at the XAUI interface. To manage the multiple
buyer-supplier scenarios, it is best to use AC coupling. If, however, a
PHY module is going to be integrated by the switch manufacturer on a
single board, then XAUI becomes the switch manufacturer's internal design
responsibility, and they should have the freedom to choose DC or AC
coupling. Just as it would be wrong to burden a pluggable module with
mandated DC coupling, it would be wrong to burden an integrated single
board design with mandated AC coupling.
Beyond this common ground, where we go from here becomes an interesting
choice. One approach would be to leave the coupling issue to the
implementers. Another approach would be to say, the popular purpose of
XAUI is to allow easy separation of switch and PHY module
responsibilities, and to allow the implementation of pluggable or
solderable PHY modules. To help achieve that purpose in a bullet-proof
fashion, we should mandate AC coupling. This is the "majority gets its
way" approach. At the moment, I favor the second approach. To make
further progress, it will help to know what others think.
> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Rich Taborek
> Sent: Saturday, September 23, 2000 12:55 AM
> Cc: HSSG
> Subject: Re: XAUI AC coupling
> OK, I asked for it... Now I'm forced to respond.
> A PHY module to ASIC connection, with the ASIC being in-line and
> eventually connected to a switch fabric, is very likely to be a XAUI
> implementation. If the module is pluggable, then it is very likely that
> the XAUI link would be AC-coupled to insure maximum interoperability.
> However, if the module is fixed, there are non negligible cost,
> reliability and performance advantages to employing DC-coupling instead
> if applicable. There is no risk. My point all along, is that
> coupling is
> an implementation detail and should not be a standard mandate.
> Your PECL example reflects information an implementer should be able to
> glean from a manufacturers data sheet. Many XAUI devices will be fixed.
> The implementer simply reads the relevant data sheets for both XAUI
> devices and determines whether or not AC-coupling is required. If
> AC-coupling is not required, the implementer may choose to DC-couple.
> The determination of signal coupling requirements is standard practice
> for chip-to-chip interconnects.
> Your third paragraph seems to describe a scenario where an implementer
> makes a bad decision to employ DC-coupling where the device specs for
> the two XAUI devices employed in the link dictated AC-coupling. I was
> unaware that the purpose of the standard was to force suboptimal
> implementations in case an implementer misinterprets device
> data sheets.
> I believe that most implementers would be incensed by such imposing
> regulations. I certainly hope that the same implementer doesn't rely on
> the standard for all other aspects of XAUI link implementation, such as
> power supply decoupling, trace layout, connector choice, via design,
> etc. to insure that their XAUI links work reliably.
> I don't understand the relevance of LVDS to this discussion, please
> I agree that if either an implementer is uncertain about DC bias or DC
> bias itself is uncertain, that AC-coupling should be used. However, you
> seem to be describing a scenario where too much uncertainty exists. It
> is highly likely that the operation of the XAUI link will be uncertain
> in this case.
> To conclude, your assumed XAUI configuration is system ASIC to
> transceiver module which exemplifies only one possible XAUI application
> and one in which DC-coupling is applicable and preferred in many
> instances. In addition your desire is to impose suboptimal
> implementations on all XAUI links in case an implementer
> happens to make
> a mistake. I have to respectfully disagree that either
> argument dictates
> that XAUI AC-coupling is technically required.
> Best Regards,
> Vipul Bhatt wrote:
> > Rich,
> > It seems to me that AC coupling deserves more to be "required" than
> > "not required".