So, I’ve noticed that the TI 84 Plus calculator’s I/O port is indeed a 2.5 mm audio jack. Could this mean that it is a standard UART serial port? No, unfortunately it is not, apparently it uses some other custom, non-UART style protocol for communications. Oh, bummer, it was so close to being convergent technology. The GnuBee uses an 3.5 mm audio jack for its UART serial connector.
Nevertheless, there is a pretty good Arduino code library to interface with the custom protocol.
20191216/DuckDuckGo ti calculator 2.5 mm audio serial voltage
20191216/DuckDuckGo ti-84 calculator 2.5 mm audio serial voltage
20191216/https://electronics.stackexchange.com/questions/51486/how-to-read-data-from-a-3-5mm-headphone-jack
20191216/https://www.cemetech.net/forum/viewtopic.php?t=4771&start=0
20191216/https://www.cemetech.net/news/2014/10/699/_/articl-arduino-ti-calculator-linking-library
20191216/https://github.com/KermMartian/ArTICL
20191216/https://www.cemetech.net/forum/viewtopic.php?t=10809