I think this is just saying SBUS inversion could be a problem.

We don’t pull the pins high or low by default and there is no inversion HW on the Blitz because it is an F7 and doesn’t need it.

You could try changing the bootloader to pull the pins high specifically.

I am using the Matek ELRS RX but I have others that also work fine.

So, it looks like I’ve managed to solve this problem.

The ELRS site describes a similar problem, but since everything works as it should on betaflight, it must be FC firmware.

I found the line DEFAULTGPIO OUTPUT LOW PULLDOWN in the hwdef-bl.dat file and assumed that if it was deleted it might help.

Here are the steps I followed. Perhaps it will help someone else:

  1. Configured the Linux environment as described here
  2. Find ardupilot/libraries/AP_HAL_ChibiOS/hwdef/BlitzF745AIO/hwdef-bl.dat file and commented out DEFAULTGPIO OUTPUT LOW PULLDOWN line
  3. Build a bootloader as described here. The required file should appear in ardupilot/build/BlitzF745AIO/bin
  4. Uploat the resulting file to the FC as described here

I could be wrong in some ways because I reached this result about a week ago. But everything seems to be correct

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Hi Boris, thanks for the sleuthing. Which pins did you have your ELRS receiver connected to? Sounds like pulling pins low on UARTs is a bad idea.

Initially I connected the receiver to all possible SERIAL ports, but none of them worked. After re-flashing the bootloader, I connected the receiver to SERIAL1