Pixhawk2 gps module

Have old one for apm 2.8

gps cable

In manual very simple scheme and it is not explained. Why need connect compass to I2C2 (scl sda) when GPS’s connector already have same pins, but there is difference in direction of pins. And there are two different I2C bus what is for what?

[details=pixhawk connector spec]


[/details]