Hi, I wanted to extend PX4 support for this protocol. But I came across an error that I was not able to connect the receiver to the FMU. More information about this issue is here:
opened 06:48PM - 11 Oct 21 UTC
**Describe the bug**
Inverted UART does not work. Is not inverted. I tried to m… ake FRsky telemetry (S.port) work. And I was unable to get in working. After some time, I looked at it with an oscilloscope. And the data from the autopilot looks like this:
It was tested with CUAV V5 nano (FMU v5 STM32F765). Telemetry was enabled in parameter to some TELEM port. (Several ports have been tested with the same result). FRsky telemetry driver started correctly in the correct mode (single wire, inverted).
https://github.com/PX4/PX4-Autopilot/blob/801ef2d5200a31e8a801a5d6b0faf0aec18be7ef/src/drivers/telemetry/frsky_telemetry/frsky_telemetry.cpp#L384-L387
Level 1 in the time inactivity is forced by autopilot. Although an internal pulldown should be connected.
https://github.com/PX4/PX4-Autopilot/blob/801ef2d5200a31e8a801a5d6b0faf0aec18be7ef/src/drivers/telemetry/frsky_telemetry/frsky_telemetry.cpp#L201-L213
After researching the firmware, I made sure that the IOCTL is written (but i don't know if it's set ). I tried to hardcode it in FRsky driver. It does not help. There are several problems:
- Is hold in a high level (Internal pulldown should be set)
- Data are not inverted (it should)
Data from the FRsky receiver looks correct and inverted. And autopilot responds to them.
Upon further research, I found that this feature is included in NUTTX for this FMU. (I didn't check it in the processor documentation). And on the forum I found a person who also deal with this issue (for another purpose): https://discuss.px4.io/t/dynamixel-servo-on-px4/22995 Therefore, I think it's a problem in some lower-level of the UART driver.
**Expected behavior**
I assume (minimally) that in the inactive state it should be in Log0.
**Connection**
I have connected S.port of FRsky receiver directly to TX pin of autopilot's TELEMx port.
**Additional questions**
- Has it been tested that FRsky directly connection to RC transceiver works?
- Is FMUv5 a supported platform for that feature (especially CUAV NANO V5)? I can't find exactly this information anywhere. But in NUTTX this property is defined for STM32F765
