[SI-LIST] Re: Diff.Pairs

RTL = Rock Transistor Logic.  Saw it in an old Flintstones episode.

Sorry, couldn't help myself ...

Todd ;-)

Todd Westerhoff
High Speed Design Specialist
Cisco Systems
1414 Massachusetts Ave - Boxboro, MA - 01719
email:twesterh@xxxxxxxxx
ph: 978-936-2149
============================================

"When did the choices get so hard, with so much more at stake?
 Life gets mighty precious when there's less of it to waste"

- Bonnie Raitt, "Nick of Time"


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
Sent: Friday, October 03, 2003 5:23 PM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Diff.Pairs


Two words, "IBM TCM".

-----Original Message-----
From: Michael_Greim@xxxxxxxx [mailto:Michael_Greim@xxxxxxxx]
Sent: Friday, October 03, 2003 2:20 PM
To: chris.cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Diff.Pairs


Hi Chris,

There were many a supercomputer that was
made with that mix of technology.  You needed
the ECL speed for the processing but the memory
was CMOS.  A great chance for noise when
converting between the two.  Unfortunately I
remember when greater than 8 mils was state of
the art.  Anyone remember DIP packaging.....8-)

Best regards and have a great weekend.

Michael

-----Original Message-----
From: Chris Cheng [mailto:chris.cheng@xxxxxxxxxxxx]
Sent: Friday, October 03, 2003 2:15 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


I thought you are old enough to remember the days when board traces are wide
(6-8 mils were state of the art) and technology don't mix (either you have
ECL or CMOS as critical highspeed design on the system but not both). It is
easy to justify tight coupling of different pairs then. It still makes sense
for many designs where differential I/O is the only thing you care (e.g.
FCAL or 3GIO subsystems or clusters).

-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Friday, October 03, 2003 11:41 AM
To: Duane Takahashi; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


More than that, it does not have any benefit.  Tight coupling of
differential pairs forces the traces to be narrower increasing the skin
effect losses.  Also, this tight coupling is going to result in good old
cross talk that actually degrades the edges.

How the notion of tight coupling of differential pairs as beneficial got
started is a mystery to me.  There are several references that show that
tight coupling is not beneficial, one of them is Howard Johnson's latest
book, at least one column he has written and my recently released book.

Lee Ritchey



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