[SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines

My first response is - huh?  
  I guess at any time, we all risk being misunderstood.
   
  Students seeking answers from all the great gurus resident on this list, see 
below for the well written response.
   
  Eric, can we save this in the archive under "attention all students" and then 
pull it out anytime one posts seeking answers for his homework/project?
   
  - Bill, always a student but long out of school.
  
Eric Bogatin <eric@xxxxxxxxxxxxxxx> wrote:
        st1\:*{behavior:url(#default#ieooui) }                Bill-
   
  You are asking the SI list to give you some specific design rules to follow 
to help you meet your design spec. I think the advice you are getting is 
basically that the precise design rule of what line width, what spacing, what 
dielectric thickness, what copper thickness to use, to achieve a particular 
target single ended or differential impedance, depends on your application and 
the tradeoffs you want between total board thickness, manufacturing design 
rules, interconnect density, routing constraints, design margin and cost. 
   
  When you ask a general question, the information you get from this email 
group is about the process to use to find your answer, not the specific 
answers. 
   
  What you are hearing from this group is that you first need to understand the 
principles behind the question you are asking, so you know if it's even the 
right question, and what to do with the answer when you get it. 
   
  Next, you need the right tools to translate general design guidelines into 
specific design rules. 
   
  If you want a narrow design margin for a target impedance, and not over 
design your products, you need an accurate analysis tool, like a 2D field 
solver.
   
  If all you need is 10-20% accuracy, feel free to use the tables and charts 
you see in various books. 
   
  You can use a free online calculator that is accurate to between 3-10%, or 
rent a 2D field solver, for about $20 an hour from the Polar Instruments web 
site that is accurate to 1%.
   
  When you see folks on this list saying, "do your homework", I think we are 
referring to understand the essential principles- check some of the books or 
classes or freely available feature articles already posted on various web 
sites, learn to use the tools already available to you, and learn to apply the 
principles and tools to your applications. 
   
  If you don?t want to do your homework so you can solve your own problems, 
which is perfectly ok, there are a number of SI-list regulars who would be 
delighted to consult for you. 
   
  --eric
   
   
   
   
  *******************************************************
  Eric Bogatin
  Signal Integrity Evangelist
  Bogatin Enterprises
  Setting the Standard for Signal Integrity Training
  26235 W 110th Terr
  Olathe, KS  66061
  e: eric@xxxxxxxxxxxxxxx
  v: 913-393-1305
  cell: 913-424-4333
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  www.BeTheSignal.com
  ***********************************************
  -----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Bill Owsley
Sent: Wednesday, May 30, 2007 10:14 PM
To: Sam.Charles@xxxxxxxx; jeff.loyer@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx; weirsi@xxxxxxxxxx
Subject: [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
   
  I went below to check what Mr.Voorhies asked for;
    "...have had a very difficult time finding exact numbers for trace 
spacing/individual trace impedence and so on."
     
    And now I don't find in any of the suggestions (or in Mr.Voorhies note) 
anything about copper weight, dielectric constant at what frequency, trace 
width, distance above or between reference planes, length matching, number of 
vias across how many layers, crosstalk coupling factor and it's effect on 
even/odd mode impedance, pre-emphasis to compensate for longer lengths at 
higher frequencies, various "cute tricks" to emulate differential signals (one 
I really like, nearly eliminates most of the above), differential drivers or 
complementary drivers or current steering drivers, differential to common mode 
conversion factor, terminations, decoupling, and so on.  All the books/sources 
referenced have all the conflicting information that Mr. Voorhies has already 
discovered.  
  For us, immersed in the culture (or lack of it on my part) the references are 
like preaching to the choir, we nod our collective heads in agreement that 
those are indeed good sources and Mr. Voorhies says to himself, Hey I can catch 
lightning in a jar, but I still don't WTF they just said, thus the reason for 
his request.  Everyday we work with digit heads, okay, digital guys, who 
connect to the dots and then look to the SI and EMC guys to make the board 
work.  ps. we work well together cuz the only dots I connect are in my 
granddaughters coloring book.
    So given the dearth of details here's one answer (of many) he can use;
    50 ohm surface traces (no vias), spaced 5 times the distance to the 
reference plane, no breaks in the plane below the traces, matched lengths to 
within 1 mil. (my layout guys can do that in about 30 seconds all day, another 
"cute trick"), and all added parts are absolute mirrow symmetric (the parts 
might need vias).  
     
     
     
     
     
     
   



 
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