[Coco] Upgrade MPI?

[Gene Heskett]

On Thursday 07 May 2015 09:34:57 Sean wrote:
> One issue I had with a non-upgraded MPI back in the day was that I
> couldn't use any terminal programs with an RS-232 pak, characters
> would get dropped and garbled with my blazing fast 2400 baud modem.

That is an entirely different fix.  You need to bridge all 4 sockets 
together at each's pin 8, then remove 3 of the 4 pullup resistors which 
are now in parallel and causing some video flicker because of the excess 
current being used to report the IRQ. In this manner you are bypassing 
the IRQ selection logic in the mpi, so IRQ's from the rs232 pack are not 
blocked therefore causing that error.  The drivers are more than smart 
enough to locate and service the IRQ without the useless blocking.

This is totally unrelated to the coco3 vs gime problem.  That is because 
of the insufficient decodeing of the address lines in the mpi when 
software tries to write to $FF7F to change slots, OR when os9 writes to 
the mmu registers in the gime at $FF9F, that write is ghosted to $FF7F, 
changing both the slot and the IRQ path, so they muck with each other.  
The MPI fix that stops that ghosted writing is a different fix.

Keeping that straight in peoples minds is, I find, a bit like herding 
cats. ;-)
>
>
> On Thu, May 7, 2015 at 8:16 AM, Christopher Barnett Fox
> <cbfox01 at syr.edu>
>
> wrote:
> > What are the "symptoms" of a non-upgraded MPI? I have one connected
> > to my CoCo 3 today, and everything seems to work fine. Is there a
> > specific operation we could perform that would demonstrate the
> > issue?
> >
> > I recall someone on the list was working on a modern MPI
> > replacement. Is that project at the point where a kit or finished
> > product is available to purchase?
> >
> > Thanks!
> >
> > christopher
> >
> > On 2015/05/06/, 6:23 PM, "Robert Gault" <robert.gault at att.net> wrote:
> > >Joe Grubbs wrote:
> > >> Through sheer blind luck I have acquired a Tandy branded 26-3124
> > >> MPI. It looks barely used, and more importantly, the OE warranty
> > >> sticker is still in tact. Because it is "virgin" I'm reluctant to
> > >> give it the CoCo3 PAL upgrade. Am I being ridiculous?
> > >
> > >If you want to use it with a Coco3, upgrade it! It really is not an
> > >"upgrade to
> > >make it 'better'" but a required change to prevent ghosting of I/O
> > > bytes into
> > >the GIME region. If you intend to use it with a Coco1 or 2, you do
> > > not need to
> > >make any changes.
> > >
> > >
> > >--
> > >Coco mailing list
> > >Coco at maltedmedia.com
> > >https://pairlist5.pair.net/mailman/listinfo/coco
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco

Cheers, Gene Heskett
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