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From: john@datacube.UUCP
Newsgroups: comp.arch
Subject: Re: VME/68020 Multiprocessor boards
Message-ID: <101200005@datacube>
Date: Wed, 21-Oct-87 17:21:00 EDT
Article-I.D.: datacube.101200005
Posted: Wed Oct 21 17:21:00 1987
Date-Received: Sat, 24-Oct-87 20:37:54 EDT
References: <44165@beno.seismo.CSS.GOV>
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Nf-From: datacube.UUCP!john    Oct 21 17:21:00 1987


>Greetings,
>...  Further, since I have
>been told that the choice of backplane can make as much
>difference as the cpu choice, please include that in
>in your comments. 
>...
>	-Mike O'Dell

I don't think it is so much the backplane, but the type of bus arbiter
used. The VMEbus spec (rev C.1) describes three types of arbitration.
They are "prioritized", "round-robin", and "single level". It allows
almost any type of arbitration. You want to read and understand chapter 3 
of the spec "Data Transfer Bus Arbitration". The VMEbus spec can be 
obtained from Motorola.

I am interested in this area, but do not yet have the depth of understanding 
I would like. We do not (yet) make a product that has an arbiter on it.

The arbiter goes in slot one of the VMEbus. A number of other functions
must also be in slot one. Most single board CPUs have the slot one stuff on
them as it is required for any system. Typically, because there is not a lot 
of real estate left after putting the CPU on the card, the arbiter implemented
is low performance, or one that will tend to give lots of bus time to itself.

Very likely, to get the best bus arbiting performance, you will have to 
buy a separate card that is a high performance slot one, or system controller
board.

I would love to have others respond with the merits of bus arbiting
schemes they have used or designed for the VMEbus.

					--John Bloomfield-- 

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