I was looking for this recently as well based on a ham fest find of a few IC-F320-6 mobile radios. Here is how I solved. Already had the OPC-478 cable. I wired up an adapter using a 3.5mm stereo inline female jack to an RJ-45 connector as follows. RJ-45 Pin 2 goes to "ring" on 3.5mm. RJ-45 Pin 7 goes to "ground" on 3.5mm. Make sure you do not short any other pins in the RJ-45 or it will not work. This adapter I created is the same thing as an Icom OPC-592. Now I just need to jury rig up an OPC-617 cables.