The power functions connector cable is very simple, there is GND and +V on one end each and the middle two cables are used to control the motors. If there is a voltage difference of +9V it rotates at full speed in one direction, -9V the other direction and everything inbetween controls the speed. Like in any other simple motor.
So what we need is a standard H-drive for any analog motors, I picked the one from mindsensors.com.
Originally designed to connect RCX motors to it, it has two small soldering holes at each of the four motor drives. From the official lego web shop I bought some more spare power function cables, cut them in half and soldered them to the motor ports. And one cable is connected to the battery box.
As the device was designed to drive motors, the NXT-G building block is just perfect. You specify the rotation direction and speed - that's it. Works very well.
And yes, I did not connect the reference voltage so I cannot daisy-chain components. Why would I want to?
