[Coco] External CHAR ROM for 6847

[Bruce Borer]

Hi Juan,
    I can think of a few ways to do it but it comes down to is it worth it.
1.) Use Dual Ported RAM. One port accesses the RAM the same way as the
current ROM with the other port accesses
the RAM via the CPU address and data buses.  This would require an
address decoder to talk to the second port.
I can discuss this further if you are interested.
2.) Replace the ROM with a larger Flash chip and program it with
several images that you can select. This is simpler
but does not allow changing the image in-circuit but would allow
several images to select.  The selection could be done
by switches or a latch that can be selected with firmware.
I have a similar circuit to replace the CoCo's ROM's with 4M Flash.
My ROM replacement has a circuit to write to the
Flash while in circuit.
3.) Use a large FPGA to replace all the circuitry and the ROM.  A FPGA
to do all that would be expensive.  This would be
an interesting project I could help with but I would not put any money
towards it.
4.) The best solution and probably the cheapest would be to upgrade to
a CoCo III, it has that feature built in.
That is what I am playing with.

Bruce P. Borer

On Wed, Nov 20, 2024 at 9:57 PM Juan Castro via Coco
<coco at maltedmedia.com> wrote:
>
> Would it be theoretically possible to use SRAM instead of ROM and have the
> charset be changeable? If one tried to design such a circuit, what would be
> a simple way to have the CPU be capable of populating the character RAM?
>
> Juan Castro
> Enviado do meu Olivetti Programma 101
> http://retropolis.com.br
>
>
> Em sex., 15 de nov. de 2024 às 03:21, RETRO Innovations via Coco <
> coco at maltedmedia.com> escreveu:
>
> > If you're still following my CoCo font exploration, I was able to get
> > some hardware working:
> >
> >
> > https://www.go4retro.com/2024/11/08/expanding-the-mc6847-deciphering-the-interface/
> >
> > Jim
> >
> >
> > --
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> >
>
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