Are all HOME PLUG devices compatible with each other? I am using 2 units now for two desktops. I plan to add 2 more desktops. Buying any brand or model of units would be good since the pair I have are used and the same brand might be unavilable. Adding units as needed would be what I would do. I know wireless phone sets work in pairs and are not cpmpatible with other brands. Added units must be compatible with the 2 already in use. I hope this restriction does not ally to HOME PLUG.