[SI-LIST] Re: How to place the AC coupling capacitor of the serdes?

Dear Liu,

Back in the early days of Fibre Channel we debated this. It turns out that
there was an additional, non signal integrity argument: Suppose that you
were to accidentally short the signal pins of a driver-connected cable to
ground; some drivers (particularly those that were similar to PECL) may be
damaged by the excess dc current. Shorting the pins of a receiver-connected
cable to ground was less likely to damage the part.

This argument notwithstanding, FC-PI mandated dc-blocking the receiver.

Regards,

Paul Levin
Xyratex
______________________

Bahlinger, Martin wrote:
> Hi Liu,
>
> Several times I searched for an answer to this question on the web and
> could not find any clue. I also asked a Gigabit serdes expert on this
> subject and all I got was: One needs the caps, but it does not matter
> whether they are placed at the receiver or at the transmitter.
>
> To have some rule on this in the PCIe spec is necessary, because quite
> often communicating devices are developed by different parties. Thus
> this rule ensures to not miss the caps on any lane. But why they chose
> TX and not RX? No idea...
>
>
> Regards
> Martin
>
>
>  -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of liuluping 41830
> Sent: Freitag, 18. Januar 2008 05:11
> To: si-list@xxxxxxxxxxxxx
> Cc: oukanghua@xxxxxxxxxx
> Subject: [SI-LIST] How to place the AC coupling capacitor of the serdes?
>
> Dear si-listers:
> AC coupling per the PCI Express Base Specification Revision 1.0a,
> section 4.3.1.2 dictates AC coupling to be placed as
> close as practical to the transmitter buffer.=20
> We consider that the AC cap is used to ensure the DC bias levels of the
> transmitted signal do not adversely
> affect the receiver,and most of time place it clsoe to the receiver
> side.Why the PCIe requires  to put it on the TX side?
> Is it differ from different serdes interface?
>
>  Thanks and Regards,
>
>                     LIU Luping
>
>   =20
>
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