Extensible PDAs of the past

Every old Palm PDA had a serial port on its docking connector. It was mostly used for synchronizing data with a PC, but the port was user accessible and used for many hardware add-ons (like keyboards, digital cameras, mp3 players, bar-code readers, modems etc.). Handspring devices went even further with hardware extensibility and introduced springboards, these modules also doubled as software cartridges. If you had, for example the camera module, you just pushed it in your PDA and camera app instantly appeared on your home screen.

Extending the SDA

My PDA had only one USB port where only power lines were connected. That was not expansive enough, so I took unused serial port on the MCU and braked it out on 1.27mm pin header together with ground and 3.3V. I used this small connector because of size constraints, It is not mechanically ideal for the job and only time will tell if it will survive harsh and dusty environment of my pocket.

With this contraption in place I added the software support for the serial port so it could be used from applications written in SVS. For the first hardware add-on I choose the U-blox NEO-6M GPS module that I had laying around and created:

Words most useless GPS

Hardware side of this add-on is quite simple: The GPS module is connected to the PDA with few wires. I quickly designed simple enclosure, printed and it works well enough. (I already designed better one, but didn't have the time to print it.)

Software was a bit more tricky because I found and fixed some bugs in SVS (design flaw in the string garbage collector took the most time and thinking). But ultimately after about 220 lines of SVS I was parsing the GGA NMEA command, showing the longitude and latitude and drawing cross marking your position on the 320px wide map of the world. It is accurate enough to guess the continent you are on.

What next

I have a DFPlayer Mini module laying around, so the next add-on will probably be MP3 player.

I am also finishing the schematics for next hw revision of my PDA. It will also have an extension connector and there will be some design features that will allow the extension to better hold on the PDA. That is actually the worst thing about the current design, the add-on can be unplugged too easily.