Date   : Tue, 05 Mar 2002 14:42:53 -0000
From   : "Richard Gellman" <r.gellman@...>
Subject: Re: Xfer

>Hi,
>Is it possible to fit a dual switcher arangement to a BBC + like those
soldisk untils you could get a long while back which allowed you to hot
switch the
>controller. i seem to remebr that soldisk was aquired by IBM.

Circuit-wise, yes. But you'd probably have to build the board yourself, as I
dont know if any such boards were ever specifically made.
The basic interface consists of an 8-bit Data bus, a 2(1770) or 3(8271) bit
address bus, and in the case of the 1770, an external drive control register
chip, a simple 8-bit 74-series register should do for that, and some address
decode logic. Then your switch just needs to switch the Chip Select line
from one chip to the other.

There is (I think, dont quote me) a circuit diagram on TBL! (if not, on a
similar well known BBC site, try the bbc documentation project) showing the
Acorn 1770 FDC upgrade board. This board simply remaps the 8271's pins onto
that of 1770, and provides an 8-bit register and some address decode logic
for the drive control register.

The register difference is because the 8271 is single density only, and has
integrated drive and head select, where as the 1770 can be switched between
single and double density, but that, and the head/drive select are not
integrated, and have to be selected externally. Note, only the density
select actually goes to the 1770 chip, the head/drive selects go directly to
the drive.

The only other problem you would encounter, is having to switch DFS ROMs, as
the 8271 and the 1770 are accessed completely differently.

-- Richard Gellman

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