XBOX XDK usb port?

Just a FYI, JVS is actually EIA-485 (RS-485, industrial grade asynchronous serial bus) and a set of application specific software protocols.

It's conveniently (confusingly actually) packaged into a 4 pin female A-Type USB connector.

 

So the "USB" connector is actualy never used as USB, but only as RS-485 but using this easy connector. also only used in the arcade dev apparently.
^^ My conclusion so far.
Do we have an example case where this "usb" 485 connector is actualy connected to a daughterboard ?

 

Not "DEV". It's used for communication from the I/O board (game inputs) to the game computer. It's purpose is simplify the harness wiring.

It's "DEV" only if you consider that it's there to facilitate the development of arcade tittles by allowing developers to access directly arcade I/O boards while testing their software ...

 

Also it's good for my project, If I connect via USB my Dev kit with baseboard USB and I can use power button from my Xbox like I/0 board and the xbox controller.