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From: cbo@hprnd.rose.hp.com (Calvin Olsen)
Newsgroups: comp.arch
Subject: Re: Re: The Future of Buses (and Futurebus)
Message-ID: <2170004@hprnd.rose.hp.com>
Date: 12 Dec 90 20:50:17 GMT
References: <43922@mips.mips.COM>
Organization: HP Roseville Networks Division
Lines: 48

In comp.arch, mark@mips.COM (Mark G. Johnson) writes:

    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".

A Futurebus+ claim to fame is:  technology independent logical protocols.
Your mileage certainly does vary, depending on implementation choices.

    On the downside, the parasitic inductance and capacitance of Metral is
    very high, which limits its applicability in very high speed (sub 15ns)
    backplanes.... From the standpoint 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.

Unquestionably, there are better connectors (electrically and otherwise).
Given that the limitations to performance in the Futurebus protocols are
primarily physics limitations, and assuming that implementations are stumbling
across the connector as THE high bar on the pareto of limitations for MOST
applications, the connector choice might be reconsidered.  To date, the
Fb+ working group members have yet to be convinced that the connector itself
is currently or will be in the near future the primary cause for implementation
concern.   I'm not trying to defend the Metral design, but we're talking
products and marketing here.

    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?

Within the physical portion of the standard (896.2), "profiles" may specify
any connector.  To date, however, most commercially-driven profiles (A/B/F)
have specified the Metral design, at least publically.  Profile D, the
"desktop" (EISA) form factor, could actually deliver better performance than
the Metral, and at significantly lower cost.  Certainly other connector
schemes could do as well or better.  As always, weighing the cost vs net
performance benefit for a given application/market is necessary.

    -- 
     -- 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}

Calvin Olsen
(not speaking on bahalf of) Hewlett Packard
cbo@hprnd.rose.hp.com   (916) 785-4291