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Re: Should we provide ISA bus support?

To: riscy@pyramid.com
Subject: Re: Should we provide ISA bus support?
From: Andreas Busse <andy@piggy.waldorf-gmbh.de>
Date: Sun, 4 Jul 93 18:33:53 GMT
Reply-to: riscy@pyramid.com
Sender: riscy-request@pyramid.com

> In general, most people want ISA.  There are one or two people
> that would rather have a simple, 'riscy' specific, cheap 8-bit bus.
> Then there is the issue of how many ISA sockets to provide?
>
>
> No external bus at all:
>
> Advantages:
>       *  Very cheap
> Disadvantages:
>       *  Make expansion rather difficult

I would say: Makes expansion impossible.

> R3000 specific high bandwidth bus (local bus?):
> 
> Advantages:
>       *  Peripherals that we add later can go fast.
> Disadvantages:
>       *  Designing a high speed expander bus is problematical
>          (lots of wave calculations)
>       *  Connectors for high speed busses are expensive.
>       *  No cards available, unless we make them.

Do we really want to invest the wheel once again ?
There are *so* many bus systems yet... I don't think
that we have the power and market to introduce a new
bus no matter how good and fast it is.

> R3000 specific slow speed 8-bit bus:
>
> Advantages:
>       *  Cheap, small connectors
>       *  Easy to provide for on the motherboard
>       *  Simple to make cards for
> Disadvantages:
>       *  No cards available, unless we make them

As above, plus the disadvantage that it will be slow :-)


> ISA bus:
>
> Advantages:
>       *  Supports many, many peripherals
>       *  Allows other options for video on the motherboard
>       *  Allows other options if SCSI is not used
> Disadvantages:
>       *  Large connectors are not the cheapest (though given the quantities
>          those things are made in, they probably are cheap)
>       *  Will could not hope to support everything ISA (such as bus-
>          mastering mode, and maybe the bus sizing stuff).
>       *  Uses lots of board space

In fact, the connectors are old-fashioned and use a
lot of boardspace. But we don't need that many slots
as a PC has, because we have most of the necessary
I/O stuff on-board. One might need two, perhaps 3 slots.
That will be sufficient for 99% of all applications.

> A full AT ISA bus implementation provides:
>       *  8-bit or 16-bit data path
>       *  Flexible throttling mechanism (automatic wait state generator)
>       *  Slave and Master DMA modes
>       *  Automatic 16 to 8-bit translation.
>       *  8.33 MHz clock rate

The discussion about DMA modes is already in progress.
Perhaps there's no need to support DMA on the ISA bus.
Comments ?

Andy

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