PerfPro Studio is a one time purchase ($129 as of this post).

I haven't tried it, but it looks like Golden Cheetah supports erg mode workouts, from the docs. It's free, so you can at least take a look.

i realized mywoosh is also free

IndieVelo is in beta and currently free

The free version of ErgVideo will play .fit files.

Golden Cheetah does it.

I heard QZ app, $5 can do this, haven't tried it yet

Check out TrainerDay @ $39.95 per year.

Just use the app associated with your trainer and manually control the setpoint based on your workout. Cheapest option outside of a garmin (free).

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Yup, this
Both Wahoo and Tacx have apps that you do this, considering these are the 2 biggest players in the trainer market there is a good chance you have one of these.

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http://www.instagram.com/cyclewise

PerfPro on PC, Golden Cheetah if you are more of a tinkerer, Breakaway on iOs (integrates with intervals.icu if you want to set up workouts), 52 different e-mail addresses for free trials on training platforms...

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Well, you can check out Flux: https://flux-web.vercel.app/.

It's an open-source and free to use web app that provides ERG control for .zwo workouts.
The only limitation is that it requires a Chrome or Edge browser in order to connect to bluetooth devices.
So iOS won't work, but anything else MacOS, Android, Windows, Linux, is fine.
It supports all major trainer protocols, power meters, heart rate straps, and some specialized devices like the Moxy SmO2 monitor.
Workout sessions are available as .fit exports, but a Training Peaks and Intervals.icu integrations are coming soon.

i tried this today, it did read watts but did not control the ERG, any ideas?