RFD900X Failsafe PPM not programmed correctly

We recently ran into a pretty critical issue with our RFD900X radios. RFdesign’s Windows programming tool does not properly set the failsafe PPM on the aircraft radio. This will result in an immediate crash if the RC radio is switched off or if the signal is lost. We tested the PPM output of the radio and saw it continued to output a PPM signal even when the ground station was turned off.

The only way to fix this is to configure the radio directly over UART using the AT&R command (or RT&R if configuring remotely over UART).

I just wanted to raise this issue so anyone else using the RFD900X radio is aware.

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Thanks for the note. Could you share a link with instructions how to program it correctly?

Sure thing. Instructions and configurable registers are shown in the reference manual.

  1. Connect using your favorite serial terminal at 57600 8N1 and an RS232 converter connected to the aircraft radio.
  2. Type “+++” to enter config mode (do not hit enter). Should return “OK”
  3. Ensure all other radios are turned off.
  4. Type “AT&R\r\n” to set the ppm failsafe on the radio . Should return “OK”
  5. Type “AT&W\r\n” to save the configuration to EEPROM. Should return “OK”
  6. Type “ATZ\r\n” to reboot the device.

If you are configuring a remote radio you use “RT&R”, “RT&W”, “RTZ” instead once you are in config mode with the local usb radio. But depending on your setup you may need to ensure the local radio is not receiving a PPM signal on the GPIO input pin or you’ll be recording an active PPM failsafe signal. You may have to turn off the GPIO input on the local radio using “ATS16=0”, saving and rebooting before you record the remote radio PPM signal. Don’t forget to turn the local GPIO input back on afterwards.

I recommend testing the aircraft’s behavior in a safe environment when turning off the radio. Each aircraft is different depending on your rc configuration but you may encounter it switch to manual mode and disarm if the radio connection is lost (since all RC inputs will go to 1000 if it does not detect the radio disconnect).