[Coco] External CHAR ROM for 6847

[RETRO Innovations]

On 11/20/2024 8:56 PM, Juan Castro via Coco 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?

Funny you should mention this, as it was the original idea that prompted
me to learn how the 6847 handles external character data.

As I figured, learning how the VDG handles external char data has taught
me some of the issues hindering the RAM idea.  Namely:

  * Address lines (save A0) are not connected to the VDG.  Thus, there
    is no way to address a memory IC from the VDG socket on the CoCo 1/2
  * There is no Read/write line to the VDG.  It's always reading data.
  * The data going to the VDG is selected by the SAM IC

So, I thought, OK, I add functionality to the SAM.  If external char
data is requested, I'll have SAM address the screen char location, to
know what char needs to be displayed, then take over the SAM
functionality and select the bitmap data for that character.

Argh, issues:

  * No data lines going to the SAM.  That means, while I can have SAM
    select the char value, I can't read it to then select the RAM needed
    for the bitmap data.
  * And, even if I could, the data lines going into the 6847 are also
    tied to reverse and alpha semigraphics.  So, I can't cleanly send 8
    bits of char bitmap data to the VDG, even if I could convince SAM to
    select the right memory location.

I still noodling on the idea, though.

Jim