The Helix 9 COMM port is for an external GPS antenna connection mostly.
The XX9 models and Helix models are not INTERLINK capable. INTERLINK has been discontinued.
All three 798's have a COMM port for an external GPS connection or INTERLINK or both with the AS YC cable.
As 44 Diesel stated, the 798ci HD SI Combo is Ethernet capable and with the same adapter that the Helix requires for connection of an Ethernet cable. AS EC QDE.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1