The Museum of HP Calculators

HP Forum Archive 18

[ Return to Index | Top of Index ]

Message #1 Posted by Robert Hollyer on 3 Apr 2008, 4:51 p.m.

I have a non-functional HP-29C that I would like to debug. Presently, it displays a brief LED flash at power-on, but not much else. Since this is my only 29C I do not have a reference calculator for comparing the IC signals. I am curious if anyone has ever located a copy of schematics or IC specifications for any of the Woodstock calculators?

Re: HP-29C
Message #2 Posted by Eric Smith on 3 Apr 2008, 10:23 p.m.,
in response to message #1 by Robert Hollyer

Aside from having more ROM and RAM chips and different power management circuitry, the 29C circuitry is similar to the 25C.

The most common electrical failure of the 29C is for someone to have plugged in the AC adapter with no battery pack or a bad battery pack, which will damage the RAM chips and possibly the processor (ACT). The only sources for replacement chips are other HP calculators of that vintage.

Re: HP-29C
Message #3 Posted by Randy on 3 Apr 2008, 10:30 p.m.,
in response to message #1 by Robert Hollyer

The only source for schematics is They are not official HP schematics, they were hand drawn by Tony Duell and represent one version, the original model of 29C. Later models made some changes to the power supply but overall the schematics are accurate and useful. There are no component identifiers or waveforms. The IC specs are not available, they are all custom HP parts. The only reference material that I am aware of can be found here

Re: HP-29C
Message #4 Posted by Robert Hollyer on 4 Apr 2008, 1:05 a.m.,
in response to message #3 by Randy

Thanks for the feedback. I requested a copy of the schematics from the HPCC group.

I tested the calculator using a Tektronix variable DC power supply and the 29C draws 0.12A with the switch in the off position and 0.32A when on. That's excessive. I suspect at least one of the HP-29C ICs is blown. Probably caused by charging with a defective battery pack. Given the prevalence of corroded battery packs, it's no doubt a common failure mechanism.

I will try to isolate the faulty IC(s). If the problem is one of the RAM chips, then I am wondering if an FPGA or similar device could be programmed to emulate the functionality of the RAM. If the problem is the processor, then the effort to recreate the IC functionality would appear to be unreasonable.

I do have a functioning 25C so I will open that unit and try to compare the signal waveforms to the 29C.

Re: HP-29C
Message #5 Posted by Eric Smith on 4 Apr 2008, 2:01 a.m.,
in response to message #4 by Robert Hollyer

An FPGA (even the low-power Actel Igloo devices) draws too much power to replace the CMOS RAM chips used in the calculators with continuous memory. The best bet for replacing those with modern chips would be a PIC microcontroller. The Clonix is an existence proof that a PIC can emulate HP memory chips.

The two-phase clock signals swing roughly from -12V to +6V, beyond the specs of PIC inputs, so you'd need to level-shift the clock inputs.

I've considered writing code for a PIC to replace essentially all of the chips in a Woodstock-series calculator other than the cathode driver and the anode driver portion of the combined anode driver/ROM 0 chip. Most of the code already exists in the DIYRPN calculator Richard Ottosen and I are developing, which already runs the ROM code of all the Woodstock models other than the 29C (and adding the 29C wouldn't be too difficult). It would need to be modified to generate the appropriate drive signals for the anode and cathode driver chips. It would be a fun project, but I doubt that I could sell enough of the chips to make it worthwhile.

Re: HP-29C
Message #6 Posted by Robert Hollyer on 4 Apr 2008, 8:46 p.m.,
in response to message #5 by Eric Smith

Thanks for the details regarding the Clonix project and the PIC chip. I am still trying to obtain a copy of the HPCC schematic CD-ROM, so that I will have a better understanding of the 29C circuit. Has anyone considered adapting standard CMOS SRAM as a replacement for the HP RAM?

If you do create a Woodstock replacement chip, let me know. It would be an interesting project, but I agree with your comment regarding the lack of profitability.

Re: HP-29C
Message #7 Posted by Eric Smith on 7 Apr 2008, 6:14 p.m.,
in response to message #6 by Robert Hollyer

To use a standard RAM, you'd need a lot of logic for the interface. A single CPLD could contain that logic. Xilinx makes CPLDs with very low quiescent power dissipation, and perhaps other vendors do as well, but it would still require level-shifting circuitry for the signals (phase 1, phase 2, SYNC, ISA, and DATA).

Some of the PIC microcontrollers are rated for operation up to 6V, so if you don't mind pushing them slightly beyond specs, they might only need level shifters on the clock phases. There are also some "HV" PICs which are rated for even higher voltages, but that doesn't help with the clock phases since those swing negative.

Re: HP-29C -- HPCC schematics
Message #8 Posted by Tony Duell on 6 Apr 2008, 5:16 a.m.,
in response to message #3 by Randy

Needless to say, the schematics I drew out are those of the machine(s) I could find. I am not really a handheld collector (I prefer the desktop machines), so I had to borrow many of the handhelds from people who were happy for me to take them to bits.

Complicated machines, like the desktops, take several weeks to trace out. I am only prepared to do those if I own the machine. I found I could do a handheld in an afternoon without problems, so I didn't mind borrowing those.

It's worth checking 'related' schematics if your machine doesn't agree with the diagrams. For example, the 35 on the CD-ROM has the old, discrete-component, reset circuit, the 45 has the one built round the custom reset/clock driver chip. I am told some early 45s have the discrete component circuit, I would _guess_ the 35 diagram would be helpful for this part of the circuit.

[ Return to Index | Top of Index ]

Go back to the main exhibit hall