External GPS module works fine on GPS2 port but not GPS1

ArduCopter
1

Hello,

I have CubeBlack running Arducopter 3.6.11 (and tried 4.0).
When I attach the external GPS (NMEA HDT and NMEA GGA) to GPS2 (Serial4) port, it works pretty well, but changing this to GPS1 port (Serial3) with the same configuration (baudrate, GPS_TYPE…) will not make Ardupilot read or detecting the NMEA message.

Anyone experience this before?
Thanks!