Audio signals in FT-900 can be found on 3.5mm TRS socket:
Serial CAT control lines and PTT can be found on 6 pin MiniDin socket:
The pinout cable for Digirig should be as follows:
3.5mm tip (DATA IN) → audio 3.5mm TRRS ring1 (RIG_AFIN)
3.5mm ring (DATA OUT) → audio 3.5mm TRRS tip (RIG_AFOUT)
3.5mm sleeve (GND) → audio 3.5mm TRRS sleeve (GND)
MiniDin6 pin 1 (GND) → serial 3.5mm TRRS sleeve (GND)
MiniDin6 pin 2 (SERIAL OUT) → serial 3.5mm TRRS ring 1 (RIG_TXD)
MiniDin6 pin 3 (SERIAL IN) → serial 3.5mm TRRS tip (RIG_RXD)
MiniDin6 pin 4 (PTT) → audio 3.5mm TRRS ring 2 (PTT)
From what I’m gathering from online sources, the serial port uses logic levels.
The link to the manuals can be found on this page.