Zwift use ant+ fe-c standart, but fe-c is bad. Ant+ require only physical device - ant+ usb dongle , and intentionally done restriction - other devices cannot communicate with ant+.
Zwift support often responce:
“Unfortunately, it is not possible to access Tacx closed system and we would not encourage anyone to do. I would suggest that you contact email@example.com as they would be best-equipped to answer your query.”
But this answer incorrect. Problem is in ant+ incompatibility, ant+ does not allow TACX to be compatible without modify hardware part.
Old tacx trainers has very simply protocol - 20 times per second tacx app transmite resistance value to brake and receive in responce current speed, RPM , resistance, heart rate and some other values. Only few numbers.
I have program that can get/set this values from brake. Only problem how to transmite this values to Zfift without ant+ libraries.
Or Zfift support only ant+ fe-c restricted protocol and nothing else?
goldencheetah for example has fortius compatibility: https://github.com/GoldenCheetah/GoldenCheetah/blob/release_3.2.0/src/Fortius.cpp - sample code.