Date   : Wed, 06 Mar 2002 00:12:22 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: Xfer

On Mar 5, 23:16, Richard Gellman wrote:
> >Not quite.  You'd have to enable one / disable the other.  The two chips
> >live at different addresses.
>
> Thats what the chip select line does. The 8271 and the 1770 both have an
> input marked as "(not)CS".
> (not) is represented by a bar over "CS". This means the chip is selected
> when that line is low.
> Normally, its connected to the Acorn IO Controller chip, which is a
custom
> chip (at least in the master, it may be "manual" circuitry in the model
B),

But we're not discussing the Master; this thread started with specific
reference to a B+.  In a B+, the two FDCs are at *different* addresses, and
that is how the two DFS's can tell whether the controller they require is
present.  The 8271 occupies &FE80-&FE83 and the 1770 occupies &FE84-&FE87
(not all addresses used in either case).   AFAIR, the same is true in a
Model B.

There's no provision for an 8271 in a Master, unlike a B+.  The 1770/1772
in a Master lives at &FE28-FE30.



-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York