Date : Tue, 05 Mar 2002 17:10:25 -0000
From : "Richard Gellman" <r.gellman@...>
Subject: Re: Xfer
>> The only other problem you would encounter, is having to switch DFS ROMs,
as
>> the 8271 and the 1770 are accessed completely differently
>
>You could probably just put both ROMs in. The 8271 DFS ROM won't do
>anything if it doesn't find a 8271 controller, and I think the 1770 DFS
>ROM acts in the same way if it can't find a 1770 Controller.
In my experience, thats not entirely true.
I believe the Acorn DFSes were able to detect the presence of the relevant
_interface_ but not chip.
i.e. Whils't devloping the deifferent FDC options for BeebEm, I discovered
that the Acorn BBC B 1770 DFS checks for the presence of the 1770 Interface
by writing a select to the drive control register, and reading it back again
and comparing the result.
So this doesn't actually go through the 1770 at all.
The Watford DDFS and Opus DDOS dont even bother, and just write to
whatever's there and hope the interface it wants is present and correct.
One thing I left out in the previous email, is if you switch the Chip Select
line between the 8271 and the 1770, you'll have to use the 1770 chip select
line as an enable to the drive control register and adress decode logic,
else you'll have a drive controller register appearing on top of one of the
8271 registers.
If you're really ambitious, you can build a similar setup for the DFS ROMs,
copying all lines to both chips, and just switch the chip select lines. This
switch would have to be either seperate from the FDC switch, or a DPDT
switch that switches both lines from the same switch device. However, if
you're careful, you should be able to get away with two different DFSes in
the same system.
-- Richard Gellman
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