HP-41 GPS Module - True or fiction?

[mfleming]

Here's a fun little project I've been playing with the past couple of months that at least provides GPS for the 41CL using its serial port. Something more along the line of the HP-25 ACT + GPS would naturally require a little more work!



Going clockwise in the above photo montage, the upper left photo shows the GPS assembly connected to an HP-41CL serial port. The 41 shows the menu of the GPS FOCAL program. The upper right photo is a close-up of the menu, with labels for Time, Latitude, Longitude, Altitude and number of Satellites. The photo below is the 41 reading back the Latitude string and processing the result. The photo below that is the latitude in the X register, and the bottom right photo is the latitude string in the Alpha register. The result for the commands is always returned in the X and Alpha registers.

The final lower left photo shows to scale some parts that might actually bring to life a GPS module for a regular HP-41. As shown by Maximilian Hohmann in a previous post, a printer module shell might hold a smaller GPS module. The board in the picture is a Teensy 3.2 SBC that might do the needed transformation from 41 I/O bus to GPS module.



This montage shows the top and bottom view of my current assembly. It consists of a Maxim RS232 interface chip, an Arduino Pro Mini SBC, and a Ublox M8N-based GPS module. All components are through-hole for easy soldering. Both the Arduino and the Ublox M8N GPS module are cheap and easy to find on TAS. Three AA batteries provide power to the Arduino, and it supplies regulated 3.3V power to the GPS module.

The logic analyzer display on the bottom shows a key contribution the Arduino makes; proper serial communication with the 41CL board. A ten millisecond delay is introduced between reception of a command from the 41CL and the sending of results. The delay is needed for the time needed between the YEXP call sending a command and a YIMP call capturing the result. A four millisecond delay between bytes sent is needed to avoid serial port overrun. Perhaps the 41CL v5 board could provide serial input buffering via a deep FIFO?

I think in the last and final iteration I'll go for the smallest feasible board, with perhaps the footprint of a four AA-cell battery holder in a 2x2 stack arrangement. Use a Teensy LC SBC for easy programming via USB port, and a GPS module of any sort mounted separately. The GPS module I'm using is battery backed for quick re-acquisition when powered back on, and can be programmed to log position, heading, speed, etc. Might be interesting to dump a log of my morning walk to a 41CL RAM page, but I'm stumped at the moment as to how I might show Google Maps on the 41 display

~Mark
(Well, not surprisingly, inserting a google drive image doesn't work, so I'm back to using attachments!)