In it, he shows an example of using all three ESP32 UARTs from an Arduino program. He calls the third port “secret” although that’s really a misnomer.

Code explanation On the first line we set a variable pinButton with an integer value of 8 (the pin number on Arduino Board where we connect the button or switch). Then in the void () function we set ...

To do so, the Arduino was set up to monitor the BUSY line on the printer’s parallel port, setting the CTS line active whenever the busy line was inactive.

DIY musical instruments are their own thing entirely, but if you're looking for a place to start, Adafruit has a guide to building your own MIDI controller using an Arduino.