It's been a while since this question was originally posted by me, but I finally got around to digging into this project and wanted to share my results.
I have a FV-101DM and a FT-901DE (same remote VFO requirements as the FT-901DM and the FT-902 series). I've seen several notes from people who thought they would directly interface, and decided to dig into this and get mine working. They will not plug-n-play. After reading the notes here and elsewhere, I studied the schematics and realized that they are incorrect and would have 12vdc being applied to the VFO output and the wrong cable being modified for operation.
The FT-901Ds have a totally different interconnection design from that of the 101ZD's. The 101ZD's have two cables between the VFO and transceiver, the 901Ds utilize one. Additionally, the FV-101Z & FV-901DM have a different pinout on VFO port B than the FV-101DM.
After a bit of experimentation, I discovered the FV-101DM can be interfaced to the FT-901DM with a little work, and retain most of the functionality of the FV-101DM's original design. Here's what has to be done:
1- A jumper must be installed from pin 7 of port A to pin 1 of port B on the VFO.
2- The 6-pin cable supplied with the FV-101DM only utilizes 3 wires (with no shielded circuits for the VFO output), as it intends for the VFO communications to the 101ZD to be via VFO port A. For this reason, a new cable with 6-pin connectors must be assembled to connect from the FT-901DM to the FV-101DM VFO port B. Wire the cable, pin to pin (i.e. Pin 1 to pin 1, pin 2 t pin 2, etc). Use a piece of small coaxial cable with the center conductor connecting pin 1 to pin 1, and the shield connecting pin 2 to pin 2. Standard unshielded wire is fine for the other pins.
3- Assemble a power cord to plug into port A on the VFO, to supply an external 12 vdc source to pin 1. Include a fuse link at 250ma to protect the circuit in the event of a problem. Connect the negative of the 12 vdc source to the cabinet of the VFO (there is no ground connection in the socket for port A on the VFO).
That's it! Mine is working great, and has all functionality except the VFO memories can no be programmed from the FT-901DM, but can be programmed from the VFO just fine. This is because there is no feedback path from the FT-901's internal VFO to the FV-101DM VFO like there is with the FT-101ZD.