Thanks Wayne for the help..
But I think you mis-read my post.. I don't plan to use the Ethernet capabilities, and will only use 1 unit on the boat. (will sell the 778c to finance the 798)
I looked at the Humminbird Website, and they don't list the temp sensor TG-W as being compatible with the new HD units. BUT the extension cable is listed for HD units. So, I can guess that it will work.
The temp sensor will be using the same data port as the Ethernet plug, So I need to know if it will work.
My main fear was the NMEA port, and it seems to remain untouched from what I have seen from pictures from the web.