Revealing the HP-82242A IR Printer Module

[ThomasF]

Update: 2024-02-20 - Some small corrections in Blinky.pdf, i.e. info about some missing registers.

Some months ago, Andrew showed us all how a Raspberry Pico could be used to read the bus signals from a HP-41, and also how to inject data on the bus so that the Pico could both read and write to the calculator.
I quickly adopted his ideas and build myself a platform that I could use to test and develop code to be able to extend the calculator, and so far I have been able to:

Use multiple serial ports
With the help from Meindert I now use several virtual serial-ports over USB to my PC.
One is showing the command line interface (i.e to configure modules, start trace etc).
Another shows the disassembly trace, one for the printer output and another I use for my virtual wand.

Trace execution
Andrew also showed how to trace execution, which I extended further. I have included a complete disassembler (no symbols yet) so I can trace execution more or less in real-time. Since the buffer in the Pico is limited, I have added ”trace-breakpoint” support which allows me to start and stop tracing on any address in the address space, so I can e.g. disable trace in known loops (like waiting for key input). Using this I can trace for minutes without missing a single instruction.

Add any existing software module
In my Pico I have 16 MB of flash, so I can download a lot of modules, and from a command line tools plug and unplug the modules, specifying which port to put the module in.

Simulate Q-RAM
Any module can be marked as RAM, so I can use tools like the Zenrom to update code or write my own Mcode. Any modules can later be uploaded to the PC.

Virtual wand
As I have shown in a previous post, I added support for the HP-82153 hardware, so I could emulate the wand, i.e. sending barcode data from the Pico to the 41 bus just as the real wand would do.
Now I have also used one serial port for this, so I can ”scan” any barcode with my mouse on the PC which then sends the barcode data directly to the Pico and in to the 41 – so now it is very easy to send any program to the calculator – just as easy as with the real wand and paper barcodes but with the mouse on the PC display instead!

X-Function/X-Memory
The RAM is extended, so I can emulate the X-Memory. This RAM is saved in flash, so the data is persistent, and I can also have multiple banks with X-Memory, and swap between different banks.

And finally the Blinky …
To further test the capabilities of the Pico, I decided to try and emulate the IR-module (since I don't have one). As this module was undocumented, I started to analyze and comment the ROM code and from that I built up a model for the hardware parts of the module. With the help of Meindert, Mike and Christoph I was finally able to emulate the complete module. I have also added an IR-led to the Pico and can now print on the HP-82240A printer from my 41 - mission accomplished!

Where to go from here ...
The Pico I have supports LiPo battery, so the setup is portable (except the configuration which is done over USB (+tracing and Wand)).
So the next step could be to support WiFi and make it completely mobile.
There are also some other interesting projects in the pipe, so keep an eye open ...

Thanks to ...
Meindert who has been very helpful! We have shared a lot of information regarding the Pico environment and he has also helped me with traces from an actual HP82242A.
Mike (who actually managed to emulate most of the module before me in his NutEm/PC) and Christoph (who used this in his V41 to add Blinky support) have also helped out to verify and find faults in my implementation. Christoph even found a fault when using the emulation together with the PPC-ROM, which later was verified by Meindert to be a real problem using a real HP82242A!
And of course Andrew who came up with the idea of using a Pico like this!

What else ...
To feedback to the community I include the ROM-listings that I have commented, and the findings I have done in the Blinky module (sorry for the formatting - it is a living document, and I hope to clean it up going forward).
With this information I hope anyone who is interested in the inner workings could use this as a start.
Please note that all comments are just my interpretation of the actual code, and that there might be both faults and mistakes, so use the information with a big pinch of salt …
Anyhow, my implementation works great, and the information has been proven to work in other implementations as well!
If you have any comments, or find any mistakes in mine, please let me know so I can update and share the documents!

Cheers,
Thomas