On the PX4 it is pretty clear where to connect the GPS unit.
But how about using the external mag sensor (“Compass”)??
Can it at all be utilized with the PX4? - Which connector ?? 
@snurre,
Here is how to connect the GPS/Compass module to the PX4: copter.ardupilot.com/wiki/common … ss-module/
Unfortunately the 3DR stand alone compass will not work correctly with the PX4 I2C port. The reason for this is that the PX4 is a 3.3vdc processor with 5vdc tolerant inputs. The voltage available on the I2C port is 5vdc which is used to power the level translator on the stand alone compass. So even if you select the 5vdc jumper on the compass board, the output of the board is always 5vdc logic on the clock and data lines.
I have actually run the stand alone compass with my PX4, however Craig Elder, 3DR Engineer, does not think it is good to run the compass 5vdc logic lines into the PX4 I2C input. He has stated that a stand alone compass that will work with the PX4 will be out in about 6 weeks.
Regard,
TCIII ArduRover2 Developer
Thank you for the wiki link, Thomas!
Note to myself: Looking for info on common hareware, search Plane AND Copter wikis…
Also thanks for the additional info.
Must admit I am tempted to try hooking up the Compass anyway, since you got that working yourself, and as you mentioned, PX4 is 5vdc input tolerant. Wonder exactly what consequenses Craig was worried about?
Things get fried? 
Or simply unreliable?
I am not the first to emphasis that the external magnetometer is very useful to find a magnetically useful spot in cramped installations cluttered with high current Cables, motors, servos, cameras…
/ Tomas