Debugging RF900 connection

I’m using a JDrones RF900 telemetry radio with a Cube running Arduplane 3.9.0 / ChibiOS and am unable to establish a connection in Mission Planner (“no heartbeats received”). Some context:

I have verified that I’m selecting the right COM port, and I’m using 57K baud rate.

The two radios seem to see each other sometimes, but only intermittently. Sometimes the LED on the rover end will be solid green for a bit and then it flashes for a bit. Sometimes it seems fairly regular – 3 to 5 seconds solid followed by 3 flashes, then repeat. Even during slightly longer periods of solid green, no evidence of a connection in Mission Planner. I’ve tried separating the two radios by several meters but no change.

The same radios used to work with the same Cube, but it was a over year ago. Nevertheless, I don’t think (memory is hazy on this) either the Arduplane/Cube software/firmware or the parameters in Mission Planner or the radio changed during that time. If there are configuration parameters I should double-check, I’d welcome the hints.

I did make one change: I switched to external power for the RF900 rather than relying on the Cube to supply it. I cut the red wire completely and connected it to the positive power supply (+5.3V) via a switch that allows me to shut off power to the RF900 independently of the main power (it’s a longer story why I did this); I left the ground wire (opposite side of the connector on the Telem1 port) connected to the Cube but spliced in a lead from the ground side of the power supply. Unless I’m misunderstanding the circuit and how the Cube talks to the RF900, I’m not sure how this could have changed the behavior.

Any suggestions welcome.

The 2 radios should get a solid link light (green) even just with power only.
Connect each one up to mission planner and check all their settings are identical.
The problem may be the power source of of one of them. On the aircraft you need to run the radio off a 5v BEC, not the 5v from the telemetry port - EDIT - I see you’ve done this. The ground (0v) needs to be connected to the radio, the FC and the BEC.

Thanks. Here’s an additional mystery:

When I went to try things again today, the radio gave a solid green light as long as it was unplugged from the Cube. As soon as I plug it into the Telem1 port, it turns red. I don’t recall seeing the red LED at all when I posted my message above. I have no idea what could have changed overnight.

Remove CTS and RTS wires from the cable that connects the RFD to the Cube. Keep gnd tx rx and power only. And as stated before power it from a BEC not from the cube.

Thanks, I’ll give that a try on Monday. As I wrote above, the problem apparently came about when I switched to BEC power; previously things had worked when utilizing Cube power, even though I knew that it was pushing the limits of safe power draw.

For my own education, could you explain why the CTS and RTS wires cause problems if left connected?

During powerup cts and rts are used to switch RFD to bootloader mode for firmware uploading. Puting a BEC to power the RFD can give power a little earlier than the startup, and CTS/RTS are undefined during the first stage of startup.