I've was debating on how to accomplish this; I wanted to add serial capabilities to the PortaStudio, and for a while I considered just adding a DB9 COM port to it, but instead I actually "borrowed" this idea from a Casio graphing calculator that I've had for a while. On the calculator there is a stereo 2.5mm "headphone" jack that actually serves as a serial port. It utilizes TTL UART which just so happens to be what the Pi uses. This is the wiring diagram that I followed:

Tip = TX

Ring = RX

Sleeve = GND

I've gone ahead and purchased a Raspberry Pi 4 (2GB) because of the limitations of the Pi Zero. (Please see the "Details" section for the synopsis)