This is some code I wrote a few years ago to control my iFit treadmill. This is for the older treadmills that were controlled via an audio cable. They used to sell audio CDs that had bursts of sound in them with a short digital signal to control the speed and incline of the treadmill.
I'm gonna write up how this worked at some point. For now, the code is its own documentation, I'm afraid.
I haven't tested this in years, so your mileage may vary