HP 9825 Scientific Desktop Computer

HP 9825 Manuals (Marc's curated collection)

Here is a collection of all the manuals I could find, plus two exceptional engineering resources: the HP Journal Article and the US4075679 US Patent. The HP Journal is a great overview of the engineering of the machine, and the quintessential, 606 pages long patent is the one and only in-depth reference we have about the details of the engineering of the 9825A. It is complete with the firmware source, a detailed description of the processor, and explanation of sections of the schematics.

Marcs' Essential Guide to programming the HP 9825

The programming information for this machine is scattered all around, which unnecessarily steepens the learning curve. But fear not, I have gathered it all in a few concise notes that will get you going in not time.

HP 9825 Schematics

HP 9825T ROM Images

HP 9825T Binary Code

For info, this is the 16 bit binary word code as it appears to the processor on the data bus, suitable for disassembly. It is not how the data is arranged on the 8 bit ROMs themselves, so the following is not suitable for burning ROMs. Use the files above for burning replacement ROMs.

HP 9825T Disassembled and Commented ROM Code by Craig Ruff

The HP 9825 patent has a complete commented listing of the 9825A code. It's not exactly what the production code of the HP 9825T is, but is quite close. Craig Ruff was able to match the two and recreate a commented version of the production code, which was invaluable when we debugged our very sick 9825T.

HP 98035 Real Time Clock Module Documentation

Programs recovered to date are listed here:

The HP 9825 supports two 8" floppy disk drives, the HP 9885 and the HP 9895. These are far faster, more reliable, and larger capacity than the DC-100 tape drive included in the unit. 

The HP 9885 is a single drive, single sided, single density floppy, and connects via a parallel 16-bit IO interface. 

The HP 9895 is a dual drive, supporting both single and double sided,  single and double density floppies, for up to 1.2MB of storage. It connects via the HP-IB interface plug-in (which must be either a Revised A or a B model, the original A won't work). 

But in order to use either of these drives, you'll need an HP 98288 Mass Storage ROM. And these are as rare as hen's teeth. And they were thought to be difficult to duplicate, since HP used proprietary mask ROMs with some very unusual address decoding, bank switching and power logic embedded in them. 

But that all changed when Paul Berger got his hands on  an equally rare SSS Mass Storage ROM, which my friend Jack Rubin had acquired. SSS was non-HP, but they were obviously former HP engineers from the 9825 development team with very deep insider knowledge. They made an upgraded Mass Storage ROM that supported the newer 5.25" and 3.5" HP-IB drives, at the expense of dropping support for the older HP 9885 drive. 

Paul reverse engineered the boards and found that SSS had used standard ICs and two Intel 2764 EPROMs to make an HP ROM equivalent. The only slight difficulty in the design is that it also includes a small bipolar 82S129 PROM, which does not contain code, but is instead used to implement address decoding. So you have to program 3 ROMs to make this work: the two 2764 EEPROMs that contain the same code as the original HP ROMs or the SSS ROMs, and the little 82S129 with a small binary image.

Once Paul had reversed engineered the SSS design, a copy of the SSS PCB was made by Adrian Nicol, and a make-your-own Mass Storage ROM clone solution became available to the community. You could flash the two 2764 EPROMs with the SSS or the original HP 98288 images. It was soone realized that if you used 27128 instead of the 2764, and slightly modified the board with a switch and a  resistor, you could burn both images on the larger PROM and select which one you wanted to use with the switch. That's what I did on my board, and I include explanations on how to do it in my notes in the files below.

The info to make the clone is available below, except for the Gerber of the PCBs, which I could not find. But KiCAD schematics are included as well as images of both sides of the original SSS PCBs, so making a new PCB should be relatively straightforward. 

I listed all the individual files below so you can browse around, but I also included a .zip file that has all the files in it so you can download the whole package with one click.