In order to simplify cabling and make the installation cleaner, I laid out a PCB that goes between the handheld microphone and the radio. It has a pair of RJ45 connectors on it and connectors for the CAT data, Mic Key, Mic input, and DC power out (+5V). I dug up a Yaesu service manual from the web and proceeded to design the board from the connector documentation shown in the schematics. The boards came back a while later and I went to test them yesterday. The voltages on several pins were showing negative 5V and weird things happened when I connected the computer.
The schematic that shipped with the radio was handy when I got the radio out of it's box, and it turns out that the signal naming on my board was flipped according to the schematic that shipped with the radio. Hoping that I had not screwed up that severely, I compared the schematic from the service manual to the one that shipped with the radio. The schematic in the service manual had the signal naming reversed! From the voltages and behavior, I believe it is reversed. I was not expecting a discrepancy of that severity! When I laid out the board, I poured copper across all of the bottom side of the board to tie the grounds together. The cutting and abuse needed to blue wire this board for testing is just too much. I corrected the schematic and then the PCB and sent it out for fabrication. The bare boards are not terribly expensive ($16US) but it adds a couple of weeks to finishing this project.
I did build up the modified USB to CI-V/CAT board that came back at the same time, and it works fine talking to the ICOM 756 that I have been using to play with FT8.