LEGO Power Function PF 2.0 specification
I'm not sure exactly what you are looking for in terms of a spec, but here's the pinout from the Eurobricks thread mentioned by Alexander O'Mara with some help from this post:
Pin 1 - Motor
Pin 2 - Motor
Pin 3 - GND (0v)
Pin 4 - VCC (3.3v)
Pin 5 - ID 1 (UART TX for active devices)
Pin 6 - ID 2 (UART RX for active devices)

The basic spec for motors is that they are given continuous 3.3V power on pins 3 and 4, voltages to drive the motor on pins 1 and 2, and pins 5 and 6 are used to identify the type of motor that is attached. For the basic motors, I believe that this is simply a known resistance between pins 5 and 6.