NX protocol is based on SPI (Serial Peripheral Interface) communication.
NX body is set as Master and Lens is set as Slave

Pin Out

• Pin 1: SPI Clock - Body (Master) generates clock signal to synchronize SPI communication. (Normal High)
• Pin 2: SPI MOSI - (Master Out Slave In) Body sends data to Lens. (Normal Low)
• Pin 3: SPI MISO - (Master In Slave Out) Lens sends data to Body. (Normal High)
• Pin 4: AF Tick - This line is used only when AF is performed. This tick commands lens report the lens status (AF location, moving status, etc). (Normal High)
• Pin 5: Lens Data Ready - (Normal High) Lens set this line LOW when lens has data ready to send Body. Once this signal set LOW by lens, body will generate SPI Clock, and Lens will transfer prepared data with the clock signal.
• Pin 6: GND (0V)
• Pin 7: Power (3.3V) - It is a power source for microcontroller in Lens.
• Pin 8: Power (5.0V) * It is a power source for motors in Lens (AF and Aperture motor)

His posts in Korean about making adapter (also contains some protocol details):

http://www.mydiyworld.net/?cat=108