first up - my problem is with an APM 2.5 clone (arduflyer) - I know some of you don't want to assist with hardware not bought from 3dr store (which is fine) so I'll make this point clear at the beginning so you can stop reading.
If you're still reading.
I'm experiencing connection problems with my APM 2.5 clone.
Some background: I received it a few days ago, connected it to the latest version of mission planner without problems and loaded 2.9 firmware on it (for Hexa). Did acc & compass calibration, radio calibration, GPS and telemetry modules. Everything seemed to work. After reading some reports of people experiencing problems with 2.9 I decided to downgrade to 2.8.1. before setting up the motors etc.
Did the downgrade and a reset. Everything seemed to work.
Then I did a few more bench tests the following day. I connected 5V from a UBEC to the board and connected from Mission Planner via the 433 Mhz telemetry radios. This seemed to work. Then I connected the GPS. This also worked but I received quite a few error messages such as "Parameter XXXX already set" (for different parameters XXXX) and eventually a connection error.
I tried this a few time with and without GPS with varying success. I ended up having the arduflyer board with just the radios on my desk and I could connect and disconnect successfully on a few occasions. Sometimes I would get the "Parameter XXXX already set" errors. After one of the tries where I received these errors I clicked on disconnect and then on connect and I didn't get a connection anymore. After a while I got a timeout saying that no Hearbeats had been received.
I clicked connect a few more times - same error. Note that the 433 MHz radio LEDs indicated that the radios were connected.
Then I tried to connect again via the USB cable. No success - same error. I haven't been able to talk to the clone board since. When I plug it into the PC it shows up as COM 4 Arduino 2560. However, I can't communicate with it through Missionplanner or the Arduiono IDE. I can't upload new firmware using MP or any sketch with Arduino. CLI in MP also doesn't work. When I try to initiate USB port communication with the board it appears to reset (LEDs A and C flash rapidly a few times, then do cylic flashing for a few seconds and finally the red LED blinks slowly).
I spent quite a bit of time reading here and elsewhere and found some threads with similar but not identical issued. I've reinstalled Mission Planner, USB-Serial drivers etc. to no avail.
Based on how it happened I am almost 100% confident that I didn't fry the board by connecting something wrongly (reverse polarity or too high voltage). I think the error is something more subtle - either a flaky component or some software problem.
If it is a hardware bug I suspect it is somewhere in the Auto-MUX circuit that switches between USB and radio telemetry.
Otherwise, could I have somehow "killed" my Arduino bootloader through a software bug? I don't see how but I do have an USBAsp AVR programmer at hand and I could try reflashing the boot loader.
I also contacted the vendor (only a couple of hours ago - so wouldn't expect a response yet) but even if they offer to fix it - due to shipping times from and back to Europe I'd vastly prefer it if I could somehow diagnose and fix this myself in the coming days.