GPS Port

D90
(Image from

MC-DC2 Connector Pinout
Good description is available http://pinoutsguide.com/DigitalCameras/nikon_d90_pinout.shtml

Communication
GPS module transmit data according to NMEA 0183 3.0 protocol to camera at 4800 bps (UART mode: 1 start bit, 8 data bits, 1 stop bit, no parity, no flow control). The communication goes only in one direction from GPS module to camera. Following commands are processed by D5100 firmware A: $--RMC $--GGA $PGRMM $NKGCS $--HDG NMEA standard defines maximum command sentence length of 83 symbols including leading $ and CR LF ending. Longer sentences or other commands are ignored by firmware.

Links

 * http://www.doc-diy.net/photo/remote_pinout/#nikon
 * http://grink.com/2010/12/05/nikon-d90-homemade-gps/
 * http://www.tronico.fi/OH6NT/docs/NMEA0183.pdf