This is an old post, preserved for reference.
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This is a preview of an upcoming project. Not sure if the name will stick, but at the moment it's called PET Exerciser (UPDATE: it didn't, it's now called 6502 Diagnostics). This started when I was repairing a few PETs and was using a NOP generator. This idea of these is the hard wire the NOP instruction onto the processor databus, the end result of that is it just steps through all the addresses sequentially, continually. That is useful to check address decoding, and see if chip enable pins are pulsing in the right way. The trouble is the 1MHz 6502 based NOP generator is a bit slow to check high address bits on a scope (at least one without digital storage), but too fast to check by eye.
UPDATE: I've decided to go with PET diagnostics. I have also added a reset button and disconnected that from the onboard reset, instead monitoring that to show faults with onboard reset circuits.
UPDATE 2: I've changed the board to just 6502 diagnostics, as I will be looking at other systems to use (KIM-1, VIC20, BBC etc.), I'll keep PET diagnostics as the on screen title for the PET version of the software. I've removed the reset switch, but kept the reset monitoring. I've added some DIP switches to allow screen mode to be set and maybe some other options. The PCBs have now been ordered.
UPDATE 3: The boards have arrived, but the PCB manufacturer has managed to mess up my design and rotated the ground pad of the main IC to short out several of the pins, including the +5V rail. This is what I had designed:
UPDATE 4: Still waiting for the boards, in the mean time, I've redesigned it a bit, with a bigger microcontroller. This now appears as a USB device and shows status in a serial terminal. This also has some analogue inputs so I can check the rise time of the power and reset pins (to check problems like I had with the Oric 1 repair recently).
UPDATE 5: I'll write this up properly once I have had time to do further testing, but the V1.1 boards have now arrived, with no pads rotated as far as I can see. The first one is now built up and being tested.
More on the testing of these new board.
The new boards