Clarification on compass operation


We have a 3DR APM 2.5.2 board upon which we have severed the required compass bridge on the APM board and have added a recommended external compass

My question is that the the board compass is still apparently been seen by the APM board is that normal ???

The External compass is definitely providing the direction, but i notice that if I rotate the APM board whilst the external compass is stationary, then the compass direction indicated via flight planner moves. It does however return to the “true” external compass setting

Is this the same as other members experience

Since your APM2.5.2 is not a 3DR product, but is from RC-Timer, it is hard to tell why the onboard compass is still possibly functional. However if the either compass is not operating correctly the APM firmware will use the Z axis gyro to obtain a heading when the vehicle is moved.
I have modified two APM2.5s for external compass use and the heading would change when I moved the external compass, but not my rover therefore verifying that the external compass was the only compass in use by the APM firmware.
I suggest that you use the Terminal mode to get into the CLI and try testing the compass to see if it is being initialized or not.
Unfortunately this Forum is for 3DR hardware only and your APM2.5.2 is not 3DR hardware. Please be more observant about posting issues concerning non-3DR hardware in the future.
TCIII Developer

If both compasses were sending data, you wouldn’t see data from either.

Probably what you are observing is that the IMU will provide yaw feedback even if a compass is not connected or stationary.

You can observe this by disconnecting your external compass, then powering up the APM and connecting it to MP. If the HUD shows the orientation of “N” (assuming your craft is not pointed N), but still changes when you rotate the craft, then it’s coming from the IMU, not the compass.

The inertial navigation system provides part of the data for yaw, the compass provides the remainder. So with no compass at all attached you’ll see yaw changes when rotating the APM.
A quick way to tell whether your compass is working, is to look at the heading after booting the APM, if it’s always around 0 deg then the compass isn’t working, if it’s different every time (having rotated the APM) then the compass is working.

(@Tom, is an APM v2.5.2 not from 3DR?)

I was working with a member recently who specifically said had an APM2.5.2 and said that he had purchased it from RC-Timer. If you go their website it says APM2.5.2. When viewing a picture of his APM2.5.2 from above several of the components on the pwb were mounted differently than on the 3DR APM2.5. Also, the APM2.5.2 has always been referred to as the APM2.5 by 3DR. The 2.5.2 designator is in very small print on the bottom of the 3DR APM2.5.
TCIII Developer