Re: CBM-II Character Set and Colour Expansion

Mia Magnusson wrote:

> It seems like one half of a 74LS74 is used for the 9th pixel already,
> and a signal called GRAPHICS from CA on the IEEE 488 handshake handling
> 6525 (U2) controls if the 9th pixel should be clear or repeat the 8th
> pixel (D0) from the font rom.
> 
> By setting the GRAPHICS signal to always enable the 9th pixel (just
> bend CA out of the socket on that triport, or do it in software), an
> additional char rom with it's address lines in paralell to the
> existing, could provide the 9th pixel data.

The way it is done on a MDA is to repeat the 9th pixel only on certain
characters. They carefully designed the character set so that the
characters that need the 9th bit set are grouped together in one block.

On CBM-II you don't have this luxury; but what you can easily do is to
attach a 16V8 with inputs from the PETSCII code and output telling when
to repeat the last bit or not. This would require only minimal circuitry
change.

> Page 2 of the B schematics combined with various other parts of the
> schematics generates lots of signals that has to have the right timing.
> This gives me the impression that the function is easy to understand
> without a timing diagram, but hard to make hardware additions that's
> guaranteed to actually work in all B machines.

If I have som efree time next week, I can attach a logic analyzer to
various signals on the mainboard and make detailed plots of everything
in time.

Regards,
Michau.