apm by default cannot do any serial connection (where multiple channel signal go through one wire) like ppm or ibus. to do that, this link might help: ardupilot.org/copter/docs/common-connecting-the-radio-receiver-apm2.html

​@@rakinrkz9010 thank you 👍👍

Thanks for mentioning. Besides, it's not actually polarity isssue, won't cause any damage of reverse polarity because the 5v pin is always in the middle. If anyone plugs in reverse direction, it should not damage it but it will also simply not work as intended.