Thursday, 28 August 2014

Commodore Pet Repair Part 6 - 8032 to 4032 Fat 40 Conversion

This is the sixth part of the repair and restoration of a Commodore Pet 4032, you might want to start at Part 1. The story so far, the 8032 board is mainly fixed. I'm still using the ROM and ROM replacement plug in board for the RAM only. I'm still working on the RAM, even will a full set of known working RAM it's still failing, so I'm leaving that for the moment.
The original Pet I'm restoring was a 4032-32N, 40 column, 32K, 12" screen, Normal keyboard. Here it is about 10 years ago, yes that is a 3.5" floppy on the side, and there was a Pentium 75 inside. I don't have the original insides from that machine, so I have been repairing a board from an scrap 8032-SK, which is an 80 column board. This is one of the later 'universal dynamic pet' boards. Universal because it can be configured for 40 or 80 columns. Dynamic as it has 32K of Dynamic RAM. The earliest 2001 Pets had Static RAM. It would be much easier if this one had SRAM, as the DRAM is currently causing my problems.
Why would I want to convert the 80 column board down to 40 columns? Seems like a downgrade? Lots of reasons.
1) It was a 4032, so I would like it to be reborn as a 4032
2) It has the normal/graphics keyboard, and Commodore never released a machine as far as I know with 80 columns and this keyboard. Some software expects a certain keyboard and reads it directly rather than using the OS routines, so may get the wrong keys. So best to stick with 40N and 80B.
3) I already have a very nice, fully working 80 column pet, in the form of an 8032-SK (thanks, James - I'm still looking after it)
4) This may be the clincher, lots of the software is designed for 40 column screens, and doesn't look right in 80 column.
There are ways to reprogram the CRTC to switch to a pseudo 40 column mode, but it seems to be just the middle 40 characters of the 80 column screen. The characters are never going to get fatter whilst the dot clock of the 74166 is tied to 16MHz.
The conversion is relatively simple, change the link from 80 to 40.
Oh, but not just this one, there are 14 in total.
I've fitted sockets so I can change back to 80 column without recourse to a soldering iron. I may even look at switching via a number of 157s or even 4066s, with a dual ROM and a single change over switch. For the moment, I think I'll be happy with just the 40 column mode.
All but one of these is marked -40- or -80-, but J1 and J2 aren't. J1 is 40, J2 is 80. The Y/Z link at the bottom is 16K/32K mode, again not marked.
In 40 column mode, 5 chips can be removed, two buffers, a latch and 1K of the screen RAM. I've left the buffers and latch in place as they were still soldered in, but removed the SRAM as it was socketed.
The editor ROM needs to be changed, so I've blown a copy of the 40n50 editor ROM, time to switch on.
Looks like 40 columns to me. This is what space invaders looks like in 40 column mode.
It's now working as a 4032. Must get on with finishing this before I get carried away playing games. I may have said this before, but next it's time to finally fix the RAM.

Update: I have now removed the five chips (UB6-8, UC6-7) that wouldn't be fitted in a 4032 and made the mod permanent.