Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!hal!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: comp.arch
Subject: Re: The Future of Buses (and Futurebus)
Message-ID: <43922@mips.mips.COM>
Date: 10 Dec 90 15:24:46 GMT
References: <36734@cup.portal.com>
Sender: news@mips.COM
Reply-To: mark@mips.COM (Mark G. Johnson)
Organization: MIPS Computer Systems, Inc.
Lines: 28

I have a complaint about Futurebus: if you want to build a really high
clockrate bus, they specified a less-than-optimum connector, DuPont's
"Metral".  Metral has several nifty features, including modularity (you
can get Metral modules for high curent power connections, coaxial cables,
fiber optics, etc etc), higher density: 2.0 mm pitch instead of 2.54mm,
and low cost.

On the downside, the parasitic inductance and capacitance of Metral is
very high, which limits its applicability in very high speed (sub 15ns)
backplanes.  Worse yet, Futurebus is "single ended" (N data bits are sent
over N wires) instead of "differential" (N databits sent over 2N wires),
so Futurebus has lots of signal-to-signal interaction, known in the
business as "crosstalk".  The Metral connector exacerbates this problem
because Metral's 2mm pitch causes large amounts of coupling --> high
crosstalk.  From the standopoint of parasitic inductance and capacitance,
MUCH better connectors are available; for example, the *worst* pin in
the RC6280's backplane bus connector has far _less_ L and C than the
*best* pin in the Metral.

Seemingly the Futurebus committee has decided that higher performance
connectors are desirable; word is out that a new revision of the
Futurebus spec will permit two other, mechanically incompatible,
connectors in addition to Metral..... and these two other connectors
have much lower L, C, and crosstalk.  Pretty appealing, no?
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086
	(408) 524-8308    mark@mips.com  {or ...!decwrl!mips!mark}