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Re: Should we provide ISA bus support?

To: riscy@pyramid.com
Subject: Re: Should we provide ISA bus support?
From: tim@ubitrex.mb.ca (Tim Braun)
Date: Tue, 6 Jul 93 10:57:37 CDT
Reply-to: riscy@pyramid.com
Sender: riscy-request@pyramid.com
> No external bus at all:

Expansion becomes much too difficult.  Let's put something on, even 
if it's a 0.1"x0.1" header strip with the system (or 3730 8-bit i/o) bus.

> R3000 specific high bandwidth bus (local bus?):

In my opinion, the difficulties outweight the advantages.  Put the
stuff that need high speed on the motherboard, and use a moderate-speed
i/o bus.  Simpler expansion card design would give more expansion cards.

> R3000 specific slow speed 8-bit bus:

IMHO, this is a viable option.  We could put the ethernet and scsi
options onto daughterboards, reducing cost for those who don't use them,
and only moderately increasing cost for those who do.  But I'm not
sold on it 100%.  For those interested in audio, access to the 3730
i/o buses via a header would allow simple design of a/d or d/a modules.

> ISA bus:

Will be useful, and a good marketing move.  I think a partial 
implementation would be sufficient, with about three slots (no more).  
Too many slots gives you loading and layout hassles.  Put SCSI, ethernet 
and basic i/o on the m/board, and you don't need much else.  Works really
well for Sun Microsystems.

A real cheap option is one slot, and source one of those "put the slots
in sideways" adapter boards if you want to use more than one ISA card.

We couldn't allow DMA mastering simply, because of the 24 MByte addressing
limitation of ISA.  ISA bus DMA slaves are simpler.

An option to consider is providing each slot with a different IRQ set,
to eliminate the old IRQ conflict problem with the ISA bus.  This is
feasible with a small number of ISA slots, at least for some IRQ's.
The problem is on those ISA boards that generate more than one IRQ, otherwise
all ISA interrupts from a slot could be routed to one IRQ on the 3730.
Just a thought.

EISA uses expensive chips to interface (licensing fees to Intel built
into prices).  And expensive connectors.  We could add US$100 to the
price, for capabilities not many require.  My opinion is no EISA.

Tim Braun                          |
Ubitrex Corporation                | Voice: 204-942-2992 ext 228
1900-155 Carlton St                | FAX:   204-942-3001
Winnipeg, Manitoba, Canada R3C 3H8 | Email: tim@ubitrex.mb.ca


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