[Coco] External CHAR ROM for 6847

[Brendan Donahe]

Jim,

I can speak in general as to how these "lowerkits" are intended to work
based on the compatibility modes I put in CoCoVGA.

My impression is that you would need 11 bits of address to your character
ROM.  The 7 LSBs of the data bus would be used as the MSBs of the ROM
address. The 4 LSBs would come from an external (non-VDG) counter which is
reset on RP and incremented on HS.  This counter would only count from 0 to
decimal 11, inclusive, before being reset.  Note that inverse video vs.
lowercase is dictated by VDG bit 6 as they are tied together in most
systems that use the 6847, including the CoCo and Dragon.

I believe that an additional external 8-bit 2 input mux is required to
optionally send in the 8 bits of data from the ROM via the VDG DD input
pins in place of the ASCIIish character code based on INT/EXT.  Said data
from memory would mostly be redirected to the ROM as address as described
above.

Note that because there are 12 rows in each character, this means with
simple addressing, 1/4 of your ROM will always be wasted.  To this end, you
would need 2kB for a single 128 character set.  My understanding is that
semigraphics 4 characters would not be affected, despite the fact that I
ignore this fact with CoCoVGA.

Does this help, or is this all info you already knew?  Let me know if I can
be of more assistance with this.

Brendan


On Fri, Oct 11, 2024 at 10:40 PM tim lindner via Coco <coco at maltedmedia.com>
wrote:

> On Fri, Oct 11, 2024 at 6:52 PM RETRO Innovations via Coco
> <coco at maltedmedia.com> wrote:
> > Anyone have knowledge of how this works, or a project schematic that
> > shows it?  Ed made a repro a number of years ago, but no schematic I can
> > find.  And, I don't see any open source "lowerkit" project to better
> > understand it.
>
> The LowerKit manual may have the Info you need.
>
>
> https://colorcomputerarchive.com/repo/Documents/Manuals/Hardware/Green%20Mountain%20Micro%20-%20Lowerkit_Manual.pdf
>
> --
> tim lindner
>
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