I haven't seen anyone bring this up yet. Imagine I have a single battery for my craft. I have the 3DR power module plugged in to the battery, and then it powers the APM via the PM port. The power module then sends the battery onto the ESC, which has a BEC that I use to power the output rail. I've removed JP1 so the 5V busses are separated. However, the current from the BEC has TWO return paths: through the BEC, and through the APM. That means there's a ground loop, and worse that the servo return current could fry the APM, or the small ground wire of the power module. I think there needs to be a jumper for the ground connection between the two busses, in the same fashion as JP1. Has anyone run in to this?
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The signal from the APM2.5 needs to have a common GND to the servo to work. you essentially have two loops with a common GND. In one loop 500mA flows from the PM and back again, in the other loop i.e. the BEC and lets say it's pulling 2.5A with servos, flows but it's balanced. If you removed the GND wire to the BEC, you now have a GND loop where the 2.5A from the BEC needs to flow back through the PM and magic smoke will appear.
If you don't have a common GND between the PWM Out of the APM and the Servo input the signal will be 'erratic' and not work properly. If you want to do what you desire, you need an opto-isolate the output of the APM to the servo signal in.
In any case, people are not seeing the issue you are raising. People are seeing issues when they don't connect the signal GND wires from ESCs
I don't think you understood what I was saying. I wasn't suggesting removing the ground wire to the BEC. I was suggesting a jumper between the ground pins of the APM's output side and the ground pins of the APM's input side.
So what about the opportunity for the servo current to go through the APM ground trace to get back to the battery? That's not a problem?