From Amateur Radio Equipment Database
The FT-8900 uses the following pinout for its remote head protocol, both ends of the connection use the same pinout.
|1||Data Body||serial data from body unit|
|2||Data Head||serial data from head unit|
|4||9V||9V power connection for head unit|
|5||Power SW||Head Power switch (grounded when pressed)|
|6||MOD||AF signal from microphone|