Several developers have gotten together to work out the serial protocol. Check our work on this wiki: https://github.com/ccutrer/balboa_worldwide_app/wiki
npm install serialport
sudo raspi-config no to login over serial yes to enable serial
As per serialport/node-serialport#1583, you must disable bluetooth: Add "dtoverlay=pi3-disable-bt" in /boot/config.txt
@reboot /home/pi/spaControl/start_hotTub.sh
Uses port 9000