Search for Shawnette's instructions on connecting a non-Invacare concentrator to the homefill.
The Invacare has a connection in the back that supplies a higher pressure (14 psi, I think) than the front connection. By the machine specifications, the homefill is supposed to have the higher inlet pressure.
However, Shawnette has instructions for a non-Invacare hookup. I believe it involves starting up the concentrator first until it is producing good quality oxygen, then start the homefill, and then connect the tubing supplying the homefill to the concentrator hose barb.
Edit: It is a YouTube video, and I'm not sure if is a corresponding written post or article -
https://www.youtube.com/watch?v=O2ESr25J0_I