The specification for USB is expected to provide you with knowledge of how it works. I made a MIDI controller after reading the MIDI spec about binary stuff and serial messages. DMX is new to me. I would read the spec. There may be ICs that handle USB on the Arduino, so read about that stuff.
ok, so after some reading, iv found that the 2650 should go to rs232 ok, but i need something like a max485 to convert to rs485 for dmx. iv seen some wiring diagrams, but iv also found these: