HP97 The journey begins

[Harald]

(04-29-2019 01:00 AM)teenix Wrote:  Hi all,

The journey might have come to a premature stop.

It seems the card reader chip will only work sometimes with the circuit I put together and can do weird things at other times. I have hooked two identical circuits up to the same chip. It will work on one, not the other. It works on a heavily modified PCB used for testing, not on a shiny new one. It worked with hook up leads from the main board to the card reader, but not when the card PCB is plugged into its main board socket. Even hooking up the CRO to monitor the data stream stops the reader chip dead in its tracks "sometimes", and this can happen too with the original HP67 CPU board connected.

It seems there is something about the internals of the card reader and CRC chips and how they interact electrically seems sensitive and the reason escapes me. I'm sure its just an interface problem and it is possible that the older circuits expect a different loading on their inputs and outputs that the CMOS circuitry I provided doesn't have. I have tried different pull up/down configurations to no avail. I can't play too hard for fear of damaging something that's hard to replace.

Also, I found the display will not work properly on one HP67. It looks fine until a certain display is showing then it starts flickering. The signals on the CRO look exactly as they should and I cannot find a logical reason unless its just that these particular display drivers are showing their age and don't like playing with newer chips. Again, it could be the CMOS interface.

The switch mode power supply I am using to give 3.75V battery, 6.2V and 5V supplies might be causing trouble but doesn't seem any noisier than the original.

All in all, given these circumstances, I cannot guarantee that the board will be reliable for different calculators.

Not sure where to go from here. I've been fiddling with this nearly every day for a few months and the cost of trialling PCBs is starting to be a bit of a drag on the economy.

The board looks good in its place though :-)

cheers

Tony

I can see an inductor on your PCB. Presumably you are generating a voltage you need by means of a switched mode power supply. It might be a long shot, but is it possible noise from your power supply is causing the trouble? Have you tried shutting it down and supplying the circuit from a bench power supply?

Other than that, I can only think of comparing the waveforms with the ones on an original 67. Maybe you can spot a difference somewhere...

Good luck, and let me know if I can be of any assistance. I would love to see this project finished!

Cheers,
Harald