PET Diagnostics

Name: PET Diagnostics
SKU: 36635
Price: 80.00 USD
Availability: InStock

A module which plugs into the 6502 CPU socket of a Commodore PET to diagnose faults.

Designed by Tynemouth Software in United Kingdom

2025-12-27T15:23:32.613Z-PET Diagnostics V2.0 angled.jpg

2024-11-20T14:11:51.765Z-PET Diagnostics V2.0.jpg

2024-11-20T14:11:51.765Z-PET Diagnostics V2.0 Showing pins.jpg

2024-11-20T14:11:51.765Z-PET Diagnostics V2.0 on 2001N.jpg

2024-11-20T14:11:51.765Z-4032 boot screen.jpg

2024-11-20T14:11:51.765Z-4032 Results.jpg

2024-11-20T14:11:51.765Z-4032 Graphics mode.jpg

2024-11-20T14:11:51.765Z-4032 Business Mode.jpg

2024-11-20T14:11:51.765Z-4032 Errors.jpg

2024-11-20T14:11:51.765Z-With replacement editor ROM.jpg

2026-02-01T20:46:27.481Z-PET diagnostics.jpg

2026-02-01T20:46:27.481Z-PET diagnostics - ROMs fixed.jpg

2026-02-01T20:50:37.659Z-Calculating.JPG

2024-11-20T14:11:51.765Z-Graphics mode.jpg

2026-02-01T20:50:37.659Z-BASIC 4 Business.JPG

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A module which plugs into the 6502 CPU socket of a Commodore PET to diagnose faults.

What is it? 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. What is supported It is compatible with the following machi…

What is it?

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.

What is supported

It is compatible with the following machines:

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

What is not supported

The PET 2001 is not supported. There are problems with timing of 6540 and 6550 chips on certain original 2001s, as well as problems writing to the 6550 video RAM. Also, if you are planning to diagnose faults with 6540 ROMs, please don't bother. Just assume they are all bad. They will all fail, sooner rather than later.

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.

Why did you make it?

I repair a lot of Commodore PET computers and wanted something to help diagnose faults, so I designed this to replace the CPU in a non-booting system and display the results on the PET's own screen.

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, and if it turns out to be stuck 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 position
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 - see elsewhere in my store.

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 known data written to the video RAM can help diagnose video faults.

No PETs or other modules are included. Just the PET Diagnostics module, assembled and tested.

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Tynemouth Software Shipping Policy:

Tracked shipping is recommended on international orders. Parcel will include full value declaration and may be liable to additional delays, tariffs or customs fees upon arrival. Please be aware of this before ordering, there is nothing I can do about it.