Unable to establish telemetry link using RFD900+

EDIT: As is typical - writing it down helped with the solution. The flow control lines on the RFD-Px4 cable were swapped. Forcing off the Flow Control resulted in an immediate connection establishment.

Has anyone managed to establish telemetry link with the recent Px4 builds using RFD900+ radios?
I can get the RF link to work, but can’t make Px4 communicate with the QGC:

  1. My RF link is up. In the QGC MavLink inspector I can see Messages #109 and #166 received (not sure where #166 is coming from, it’s not part of a Common MavLink dialect). Local and remote RSSI is way above noise floor.

  2. QGC is connected to the radio over USB. Px4 is connected over TELEM1.

  3. Serial connection parameters are checked and double-checked.

  4. Mavlink status shows a single message received on the Px4 side. However, I’m surprised that TxRate is 0. I would expect Autopilot to be streaming at least Heartbeat.

instance #0:
mavlink chan: #0
flow control: ON
tx: 0.0 B/s
txerr: 0.0 B/s
tx rate mult: 0.850
tx rate max: 1200 B/s
rx: 0.0 B/s
rx loss: 0.0%
Received Messages:
sysid:255, compid:190, Total: 1 (lost: 0)
FTP enabled: YES, TX enabled: YES
mode: Normal
MAVLink version: 1
transport protocol: serial (/dev/ttyS5 @57600)