HP-97S Pod Schematic

[Katie Wasserman]

I'm working with a friend on repairing a broken 97S pod, I also have another 97S with no attached pod and am considering building a replacement pod. I've mapped out how it interfaces to the calculator and it's fairly complicated, it does some partial instruction decoding and uses all the basic control lines inside the calc. There are some 45 mostly 4000 series CMOS chips on two 4-layer boards inside the pod so it's not that easy to reverse engineer but it is easy to find replacement parts for it. Does anyone have a schematic for this?

Thanks,
Katie

[Eric Smith]

Tony Duell reverse-engineered schematics, which are on the HPCC Calculator Schematics CD-ROM.

[Katie Wasserman]

Eric,

Thanks! I forgot to check those, I got them several years ago and filed them away. Tony did a amazing amount of work tracing all those connections and they're quite helpful in tracking down specific problems.

It sure would be nice to see them woven together so that you can get a feel for the whole circuit.

-Katie

[Thomas Chrapkiewicz]

Does anyone know how to determine (via the serial number) whether a HP97 can accept external wiring - like for the 97s 'pod'?

Are the ROMs in the 97s the same as in the 97?

Thanks,
TomC

[Katie Wasserman]

The 97 and the 97S are identical as far as the ROM's are concerned. The logic in the pod does all the special work by reading the control lines and placing the data on the bus at the correct time. Similarly, it partially decods the instructions to figure out when the flags are set or cleared.

The circuit board in the 97S differs slightly from the early 97 models to allow the connection of the pod easily using a 16-pin IDC header. I believe that HP changed the boards for all 97 calculators once the 97S was released.

There should be a cutoff date after which all 97 calculators are convertible to 97S calculators. If you figure this out, the serial number should be all you need to look at. My guess is that it should be something around 1730Axxxxx and after. This, of course, is the serial number etched into the plastic not the one stuck on when they added the pod.

[Thomas Chrapkiewicz]

Thanks for the details. I was not aware that this was how the pod interface operated.

I do not have a 97s; just a few old 97s and was wondering how difficult it would be to make one a simple controller.

I will have to get the schematics and understand the logic further.

Regards,
TomC

[Katie Wasserman]

I was thinking about that myself, it's not simple. Tony's schematics will be a help certainly. There are 40+ mostly 4000's series IC's in there and some resistor networks. I suppose you could just follow the schematics but they are hand drawn and only show a tiny fraction of the wiring on any one page.

I think a much better/more interesting approach would be to figure out the control lines that the 97 uses along with the flag set/clear instructions and create a more usable interface in firmware on a PIC, AVR, whatever you like. There aren't too many control lines but you definitely need a logic analyzer. I say "more usable interface" because the pod implements this rather awkward -- in today's world -- multi-digit BCD number capture. Almost anything else would be more general purpose, but with a firmware solution you could have several different interfaces implemented: straight binary, I²C, USB, etc..

If you want a partner in this endeavor let me know and I'll do my best to help. I think it would be of value to everyone who likes the 97.

-Katie

[Thomas Chrapkiewicz]

Thanks. I've done a bit of work with some PICs, so I was considering this approach.

I have found that the 97's that I have are both after date code 1812, so I presume that the pinouts are available. After I determine the operational status of these, I will then open one and study these lines. I have many questions and must do more studying - and this project is certainly not right at the top of my priority list.

Thanks again for your info.

TomC