PET "Fat 40" 4032 RAM expansion, 80-col & IDE mods, etc.

[MikeS]

Very nice, Matt!

I did the same thing many years ago, but in the other direction, i.e. convert an 8032 to a 4032; much easier, and I didn't replace the ROM either, just used an external N type keyboard.

I did do one thing differently: after removing the 8032 jumpers I inserted header pins in all the jumper holes and used jumper blocks and wires. That made it easily reversible, and since I had a 2001 as well it was indeed returned to 8032 mode and stayed there, especially after I got a 4032. Also, when I did it I planned to add a few chips (or even a couple of multi-pole relays) which would connect to those headers to make it switch-selectable, but that never got off the drawing board.

Lots of fun.

 

[sjgray]

Also, when I did it I planned to add a few chips (or even a couple of multi-pole relays) which would connect to those headers to make it switch-selectable, but that never got off the drawing board.

I would like to build a switchable circuit myself. It would be nice if it could be software switchable. Plus the Editor ROM would need to be modified to support both 40 and 80 column.

Steve

 

[MikeS]

I would like to build a switchable circuit myself. It would be nice if it could be software switchable. Plus the Editor ROM would need to be modified to support both 40 and 80 column.

Steve

I think the only thing that might be tricky is adjusting the screen size/location if it's different enough to be ugly. The ROM should be easy, esp. if you start with a Fat 40 (thanks to Matt); my 2001 has a ROM that can switch between BASIC 1 and BASIC 2/3, same idea. Presumably you could use a bit in one of the VIAs to do the switching. What would you pick for the actual switches/multiplexers?

 

[sjgray]

I think the only thing that might be tricky is adjusting the screen size/location if it's different enough to be ugly. The ROM should be easy, esp. if you start with a Fat 40 (thanks to Matt); my 2001 has a ROM that can switch between BASIC 1 and BASIC 2/3, same idea. Presumably you could use a bit in one of the VIAs to do the switching. What would you pick for the actual switches/multiplexers?

I think rather than switching ROMs, I'd want to re-write the Editor ROM to support both 40 and 80 columns. Then I can add support to change modes with ?chr$(something) which would switch modes and adjust the CRTC registers to get the best video in both modes (perhaps chr 130 and 132 could switch as they are unassigned in both PET and C64/VIC).

Three 74LS157's should work for the address buss switching (10 lines) and clocking (2 lines). The CA1 line looks free to use as a select but it's also available on the user port so it might cause incompatibilities.

Steve

 

[MikeS]

I think rather than switching ROMs, I'd want to re-write the Editor ROM to support both 40 and 80 columns. Then I can add support to change modes with ?chr$(something) which would switch modes and adjust the CRTC registers to get the best video in both modes (perhaps chr 130 and 132 could switch as they are unassigned in both PET and C64/VIC).

Three 74LS157's should work for the address buss switching (10 lines) and clocking (2 lines). The CA1 line looks free to use as a select but it's also available on the user port so it might cause incompatibilities.

Steve

Ah, well, you're more ambitious than I was; sounds like a worthy project indeed. Great idea to use a pair of screen codes instead of POKEs.

I didn't look at the details this time around but for some reason I had the idea that some of the switched lines were bidirectional; obviously not, so a '157 sounds like the answer all right.

By any chance could you use an I/O line that's used for the second cassette? Not much use for that any longer.

 

[ayates]

I did the 40/80 column mod to my PET back when the Torpet article was first published. While I initially had a moveable header strip for the jumps (a piece of breadboard with every other joiner pulled out) I switched it over to fully electronic switching. See attached photos. I can dig out more detail if anyone needs it. The mod still works, I powered on the PET last night after it sitting dormant a good 20 years and I could flick back and forth between 40 and 80 columns.


Allan.

 

[MikeS]

Nice job! Exactly what I had in mind; got as far as installing the headers and pins so I can switch by moving the jumpers, but then I picked up a 4032 and it didn't seem that important any more ;-)

 

[sjgray]

Yes, nice work. I wonder why Commodore never thought of doing that? A dual 40/80 machine would have been a nice upgrade and would have kept compatibility with all the 40 column software already available.

If you have schematics that would be great!

Steve

 

[sjgray]

This might be a good time to mention another project I have started...

If anyone is familiar with the CBM-II High Profile machines you will know that they have 8x14 pixel characters, which give them a nicer look. I designed a small adapter for the PETs character generator ROM that shifts the bits around a little (for 16 bytes per character). A couple wires to the CRTC chip and some re-programming should be all that is needed to have the same capability on the 4000/800 series PETs. I haven't built the adapter yet, but if anyone is interested I will post the info on my UltraPET web page (http://www.6502.org/users/sjgray/projects/ultrapet/index.html).

Steve

Steve