J2 looks like serial port
- I think J2 on the board is a TTL-level serial port (with power+gnd).
I've just measured the pins on an oscilloscope. Pin 3 is definitely
sending out serial data (TXD). Pin 1 looks like 3.3V, and pin 4 is
GND. I would guess that pin 2 is RXD.
I'll wire-up a header and plug-in a TTL->RS232 level shifter and see
what I find.