PET Diagnostics Module

This is a board which plugs into the 6502 CPU socket on a Commodore PET computer to diagnose faults with the system ROM or RAM.

It is compatible with the following machines

• 9" PET - 2001*, 2001N, 3008, 3016, 3032, 4008, 4016, 4032
• 12" PET/CBM - 4016, 4032, 8032, 8032-SK

* There are problems with timing of 6540 and 6550 chips on certain 2001s, consider this mode unsupported.

It should be fine on any PET with 2114 video RAM, but does not get on with the 6550 video RAM on some PET 2001s. If you have a PET 2001 with 6550 video RAM, you will need to replace that with something like this - https://www.sellmyretro.com/offer/details/64351

It can be used on CBM 8096 and 8096-SK, but the 64K board must be unplugged first and the system tested as if it were an 8032. It cannot be used on the CBM 8296 or any CBM-II series machines.

80 column mode is supported on 8032 type machines, and the CRTC is initialised on 4000 and 8000 series machines. DIP switches on the module select the machine type.

What is tested

• The initial screen show the state of the address or data lines when floating, and when pulled high, to see if any are stuck low. Also displayed is an approximate time the reset line is held low.
• Next the RAM is tested. Results are displayed split into 8 blocks (sized 1K-4K depending on memory size), and each is separated into individual bits, helpful to narrow down to individual RAM chips.
• The video RAM is tested before the results screen is drawn, and is also displayed by bit
• The ROM chips are then checksummed and compared with a list of known ROMs. This is repeated sixteen times per chip, and an error is shown if the result is not consistent.
• When complete a sounds is made through the piezo buzzer (on later PETs).
• After the main results, the screen is cleared and a character map is displayed, in both business and graphics mode. This can be used to identify bad characters in the character ROM.

Notes

• This will not actually fix the PET, it will help identify faulty parts in order to repair the PET. If a large number of faulty ROM or RAM chips are detected, it may be easier to replace them all with one of my PET ROM/RAM replacement boards.
• The results are displayed on the screen of the PET. This needs the video circuitry to at least be partly working. However, even if there is a fault, having good data written to the video RAM can help diagnose video faults.

PET Diagnostics User Guide V2.0.pdf [1010.18] KB