Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!mips!hal!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: Re: 60 ohms considered harmful
Keywords: 75 is dangerous too, and 40 is Right Out
Message-ID: <33982@mips.mips.COM>
Date: 28 Dec 89 07:34:47 GMT
References: <33863@mips.mips.COM> <17660039@hpfcdj.HP.COM>
Sender: news@mips.COM
Reply-To: mark@mips.COM (Mark G. Johnson)
Organization: MIPS Computer Systems, Inc.
Lines: 35


   >>  Seen on page 33 of the December 18, 1989 issue of EE TIMES
   >>  
   >>     ... Augat product manager Mike Prisco ...
   >>
   >>     "All high-speed logic is going to need 50 ohms"
   >
   >
   >Yup.  And clearly in the context of high-speed logic requiring
   >controlled-impedance PC boards and connectors.  So what's wrong
   >with that?


Yow, lack-o-smileys lamentably impedes universal understanding (again).
As C programmers, ask yerselves "why is there a magic number (50) embedded
up there?"

That's the joke, ladies und gents --- the quote above, read contrapositively,
says, "if it isn't 50 ohms then it cannot be high speed logic".  When stated
this way, do you agree?  Is 60 ohms truly a not-high-speed characteristic
impedance?  How about the Cray-1 with its 90 ohm characteristic impedance
"mat"?  (Or for that matter, the CDC 6600 and its 75 ohm coax).  What about
the folks at Tektronix who claim a Futurebus+ backplane will sustain 500
MBytes/sec (note: Futurebus+ != 50.0 ohms).

Why does the number 50.00 inspire such closed-minded religious zeal?
Should we indeed publicly preach that 60 ohms is considered harmful, and
40 is Right Out??  Are Cray and CDC either badly-designed, or not-fast??


Letting out a year-end harrumph,
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}