Re: Changing the ASCII characters

[Rubywand <rubywand@swbell.net>]

Hector Gonzalez writes ...
> 
> Hello!!
> 
>  Somebody knows how to changing the ASCII characters in the APPLE IIe or in
> a Multitech MPF-III, for example, I need to change the @ for a character
> that I invented.
>

     As Nathan observes, what you want to do is swap in a custom
character generator PROM for your Text display. We did this when the
keyboard and monitor ROM were modified to support lower case on out II+.
Naturally, we also added several neat characters-- like a diamond,
square, big dot, an undeline that can actually be used to underline
text, and a hero figure for adventure games. (If you ever download our
version of Disk Muncher, the reason some of the display looks kind of
bland is that it was designed to use our II+ character set.)

     Doing your own character generator PROM (called the "Video ROM" on
a IIe) should not too be difficult. It looks like you'd use a 2732 or
2532 PROM. Address lines A3-A8 appear to select 1 of 64 characters--
probably A8 is SHIFT used to pick upper or lower case. A0-A2 appear to
select which row of dots in a character will be sent; probably, for 000,
you are picking the top row.

     I don't know about A9 and A10-- maybe a way to select INVERSE and
mouse text? It looks like, once you have selected a character and the
particular row of dots in the character, that 8 dots (logic 1's and 0's)
appear at PROM outputs Q0-Q7. These are broadside loaded into a 74166
shift register and shifted out one at a time to your display. PROM
output Q0 appears to correspond to the left-most dot position of a
character's dot row.

     A good approach is to remove your current char gen ROM or PROM--
desolder if necessary and install a socket-- and place it into a
Promming device which allows reading values for each address. Map
these-- i.e. dot-out the characters on graph paper-- and you should soon
see which Addr lines and Output lines do what. (For example, you could
start by setting A9, A10; then, set A3-A8; then, go scan the rows
top-down via changing A0-A2 from 000 through 111.)  You will be able to
pick which characters to change and know which values to program into
your custom Text display PROM.



Rubywand