RFD868+ modems stuck after boot although green leds are both on

I use the RFD868+ modems together with PixRacer and Arduplane Firmware 4.1.2 for our search and rescue drone https://searchwing.org. I tried the RFDesign firmware 1.91 and the open source firmware from tridge version 2.2 from here: ArduPilot firmware : /SiK/stable. One modem is attached via a FTDI usb cable to a laptop. The other modem is attached via UART with the PixRacer. The modem is powered from a separate power regulator, i.e. not the regulator that supplies the PixRacer. CTS and RTS lines are not connected. (Same for connected RTS/CTS).

After boot of the drone and laptop both modems show a green continous led indicating that both modems “see” each other. In maybe one out of five cold boots, sometimes more often, the initial parameter download is stuck due to heavy packet loss. So some packets come through but “most” not. If the boot is “ok”, then the parameter download is very fast. If both modem have booted “o.k.”, then all subsequent data transfer work without problem. The situation can not be resolved by rebooting only the drone or only the laptop. Both have to be switched off and the on again for a next try.

As the green lights are on on both modems the general connection is working. The packet loss could be explained by a problem with the UART or the framing. When I use the mavproxy GCS then “rubbish” packets are displayed. Normally we use QGroundControl.

We experience this problem for quite a long time with different modems at different locations with different PixRacers. We always reboot but this is quite annoying.

Does/Did anybody else experience this kind of modem boot problem?

Is the new rfd868x modem generation based on the ARM processor also affected by this?

I havent experienced that problem, but what I would do is

Use only the correct/latest RFD firmware
Set these in Ardupilot
BRD_SER1_RTSCTS,0 (assuming the radio is in Telem1)
TELEM_DELAY,2 (time in seconds to delay radio telemetry)

Let us know if that improves reliability

What are your RFD radio settings? (screenshot is good, either from MissionPlanner or RFD Tools)

Hi Shawn, thanks for your ideas. I think I can reproduce the boot problem without Arduplane. I have a description in our bugtracking:

So I tested only with FTDI cables, RFD 1.9 firmware on RFD868+ modems and with @tridge mavtester.py test program.