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.
Icom Programming Software – Download Icom Handheld Radios Cloning cables not marked with a 'B' can only be used on Windows 7 or earlier. Cables with a 'B', can be used on Windows 10, 8, 7, Vista, and XP drivers. Icom CS-F300 CPS (v2.1) Programming Software for IC-F320/F420.
Now I just need to jury rig up an OPC-617 cables.
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