[SI-LIST] Re: Which layer is better for GHz signals
- From: "Robert Sefton" <rsefton@xxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 4 Aug 2005 21:38:26 -0700
Is there anything about Fibre Channel that's NOT a PITA? How did such a
bastardized standard ever get such a foothold in the market?
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chris Cheng
Sent: Thursday, August 04, 2005 2:45 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Which layer is better for GHz signals
I have to agree.
There are times I wished I can unsolder the 150ohm terminators on those =
drives, replace them with 100 ohm and get it over with.
Fortunately most FCAL PBC or switches are programmable to 100 or 150 = ohms
so you can have a mix bag of 100 and 150 between your disk drives = and your
backplane.
Having a mix 100 and 150 differential impedance can "drive" your PCB =
vendor nuts though.
-----Original Message-----
From: Paul Levin [mailto:levinpa@xxxxxxxxxxxxx]
Sent: Thursday, August 04, 2005 1:48 PM
To: Ravinder.Ajmani@xxxxxxxxxxxxxx
Cc: Chris Cheng; Dan Bostan; si-list@xxxxxxxxxxxxx;
si-list-bounce@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Which layer is better for GHz signals
Dear All,
Long before the standard moved from 150 ohms to 100 ohms, and I believe that
only started with
4 GBd Fibre Channel, many designers had figured out that it would be better
to keep the lines down to 100 ohms and thus match the intervening connectors
rather than have loads of 150-100-150 ohm discontinuties. Sure, this meant
an overall mismatch penalty, but the lower reflections generally made it
worth it to do so.
Regards,
Paul
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