Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.arch
Subject: Re: The NeXT Problem
Message-ID: <5025@cbmvax.UUCP>
Date: 18 Oct 88 17:24:31 GMT
References: <1249@cfa.cfa.harvard.EDU>
Organization: Commodore Technology, West Chester, PA
Lines: 32

in article <1249@cfa.cfa.harvard.EDU>, ward@cfa.harvard.EDU (Steve Ward) says:
> Summary: NeXT comments

>> 2) Non-Standard NuBus Implementation: A small company like NeXT can't hope to
>>    create a competitive 3rd party board market for a non-standard bus.

>   I am unsure of the situation here, but I have heard that the "MAC"
> NuBus is nonstandard and the NeXT implementation conforms fully to the
> NuBus standard.  This is an area that needs some clarification.  Any
> NuBus experts care to clarify this topic?

I'm no NuBus expert, but I have read quite a bit on the topic.  The most 
non-standard thing Apple did was change the form factor of the card from it's
original Eurocard form (what's in the TI explorer, and probably in the
NeXT) to a PC-ish form.  I believe this form factor has now been adopted in 
some official capacity.  The other thing they did was add a Non Mastered 
Interrupt line to each slot.  Standard NuBus interrupts are weird; the board
issuing the interrupt becomes the bus master and bangs on a chunk of memory
that belongs to the board it wishes to interrupt.  This does make sense in
a system of multiple masters, but not so much on something like a Mac II,
where the motherboard CPU is basically the default master (though there is
no real default master on NuBus).

The NeXT machine changes the NuBus clock from 10MHz to 25MHz, and the bus
drivers from Bipolar to CMOS.  That should be sufficient to render it
incompatible with any existing NuBus cards.

> Steven M. Ward  ward@cfa.harvard.edu
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
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