Date   : Tue, 05 Mar 2002 18:53:16 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: Xfer

On Mar 5, 14:42, Richard Gellman wrote:

> >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.

> 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.

Not quite.  You'd have to enable one / disable the other.  The two chips
live at different addresses.

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

You don't have to do that, and in fact you'd not want to.  Each DFS can
recognise it's own controller, and will remain dormant if the controller
isn't present at boot time.  You wouldn't want to disable the 1770 DFS
completely as it includes some of the sideays RAM code, and probably the
Tube code (though if your 8271 DFS was actually a DNFS ROM, that includes
Tube code too).

All you'd have to do is ensure the disk controller you *didn't* want to use
was disabled, then do a reset.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York