I figured out how to use raw control of the TI Link Port on a TI-84+ graphing calculator in order to implement some Z80 assembly routines that could talk to an Arduino using regular asynchronous serial. Once I got that working I used it to make this demo. The touch screen is a simple resistive type panel pulled out of a junked Nintendo DS. I wrote the simple drawing program in TI-BASIC. It called programs written in assembly that executed my serial send and receive routines. Communication between assembly-land and BASIC-land happened via the Ans system variable.
The original full code is lost to the sands of time, but here's what I was able to recover: https://github.com/jmptable/ti84plus-serial