[Coco] grey MPI 26-3024 - how to tell if upgraded

[Darren A]

On Sun, Feb 1, 2015 at 2:52 PM, RETRO Innovations wrote:

>
> So, is there a link to a note that explains why the PAL changes were not
> backward compatible? As I understand it, the Coco3 MPI PAL allows
> addressing only ff7f, but the earlier MPIs would allow ghost locations.
> Why would disallowing the ghost make the unit not usable with Coco 1 and 2
> machines?
>
>

I have never experienced any problem using my upgraded 26-3024 MPI on a
CoCo 1 or 2.

The PAL upgrade does not just affect ghosting of FF7F.  It causes all
addresses from FF80-FFBF to be excluded so that the data bus transceiever
is not activated when you read from that area.  Before the CoCo 3 those
were legitimate addresses for external hardware. The GIME took over that
range so the MPI must leave the data bus tri-stated when reading from there
to avoid contention.

- Darren