"The 12c port on the top of the flight controller amp2.8 is for 12c OSD, and the port on the side of the flight controller right next to the gps port on the top of the flight controller, is for external compass 12c port..
Check out this link, scroll…"

"The amp 2.8 the compass 12c port is on the side of the board next to the gps port on top of the board, I at first was plugging into the 12c port on the top of the flight controller, that 12c port is for OSD.. 
Just this link out, scroll down to the…"

"Hello
 Did you remove the jumper next to the gps port, on the board. The weird thing that my amp 2.8 the compass would only work if I didn't plug into the 12c port, but being plugged into the gps port on the side of the board,,"