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2.1.9 is more than 4 years old, it's possible some of its dependencies have been deprecated and removed from some recent distros.

I understand that is quite old However, there are many people still converting and flying with FlySky TH9X / Turnigy 9X radios to open tx that use linux as their os. OpenTX 2.1.9 is the recommended version to use with 9X radios due to the avr radio bug: https://www.open-tx.org/2018/02/26/avr-pull. Can I compile OpenTX 2.1.9 for a current linux system to get it to work, or would there still be dependency issues? I have access to a windows os computer, but I would prefer to be able to program my 9X radios with Companion 2.1.9 on Linux.

Well in true linux fashion if a dependency isn't part of your distro you can always build it yourself.
It may be a major pain, but that comes with the linux world...

I spent several days trying to build dependencies for OpenTX 2.1.9 and gave up.

I ended up installing er9x on my Turnigy 9x Radio instead of OpenTX 2.1.9. I used eepe built for 64 bit ubuntu Linux by user rbecq usign the file is located in their post: https://openrcforums.com/forum/viewtopi ... 36#p152650

This is great stuff! Hope the Spektrum Forward programming features could be completed soon.

Hi, is 2.1 still available to download? I'm trying to update a Turnigy 9x with 2.1 using Companion 2.1.9 and all attempts at downloading it result with "Compilation server requires registration, please check OpenTX web site".

Spent a fair amount of time on the website, but couldn't find anything regarding registration.

Many thanks

No such messages exists in companion as far as I know, and it works here. A screenshot of the actual error might help.
You might need to replace the libs as mentioned here:
http://www.open-tx.org/2021/10/01/companion-downloads