Re: CBM-II Character Set and Colour Expansion

Den Mon, 16 Jul 2018 14:59:39 +0200 skrev Michał Pleban
<lists@michau.name>:
> 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.

Interesting.

> 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.

Would this really be worth the effort in comparison with just adding an
additional eprom though?

A plus with using an extra eprom is that the eprom would be the same
kind that is already needed so there is no risk that anyone building
this thing will be hindered by lack of tools to program  a 16V8. (But
yes, almost all eprom programmers probably can program a 16V8 too, but
still).

> > 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.

That would be nice, that way we can compare if the theoretically
calculated timings match actual timing on a real computer.

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