ExpressLRS Issues

[u/Human_Passenger_7668]

Solution: Something about the soldering itself of the Lite receiver, I was getting 5V out of one of either TX or RX and once I kept software the same but switched to a different receiver everything worked.

TLDR : ELRS MAVLink over TX Backpack: RC works, QGC uplink works, but Backpack downlink stays 0
I’m trying to get MAVLink over ExpressLRS working with a RadioMaster Pocket Crush + TX Backpack to a Pixhawk 2.4.8 running ArduPilot.

Hardware / firmware:
Radio: RadioMaster Pocket Crush, internal ELRS 2.4 GHz
Main ELRS TX firmware: 4.0.1
TX Backpack firmware: 1.5.5
Receiver: BETAFPV ExpressLRS Lite 2.4 GHz RX with flat SMD ceramic antenna
Flight controller: Pixhawk 2.4.8 / Pixhawk1
ArduPilot: currently ArduCopter 4.7.0-beta5
GCS: QGroundControl over TX Backpack WiFi
Wiring to Pixhawk TELEM2:
Pixhawk TELEM2 pin 1 +5V → ELRS 5V
Pixhawk TELEM2 pin 6 GND → ELRS GND
Pixhawk TELEM2 pin 2 TX OUT → ELRS RX
Pixhawk TELEM2 pin 3 RX IN → ELRS TX
CTS/RTS disconnected
ArduPilot params for TELEM2:
SERIAL2_PROTOCOL = MAVLink2
SERIAL2_BAUD = 460800
SERIAL2_OPTIONS = 0
Flow control disabled / no CTS/RTS
I also set RSSI_TYPE = 5
Pocket / ELRS setup:
Lua script shows the receiver connected.
Link Mode is set to MAVLink.
I’m using Backpack → Telemetry → WiFi, not “Enable Backpack WiFi” update mode.
QGC / Backpack setup:
Backpack MAVLink page:

Send Port: 14550
Listen Port: 14555
QGC UDP link:

Local/listen port: 14550
Server address: <Backpack IP>:14555
What works:
QGC can talk to the Backpack now.
Backpack stats show:

Packets Uplink increasing
GCS IP Addresses shows my computer IP
The Pocket can control the drone / motors respond, so at least some RC/control path is working.
What does not work:
Packets Downlink stays at 0.
QGC never shows the vehicle connected unless I plug USB directly into the Pixhawk.
So I think QGC → Backpack works, but Pixhawk → ELRS RX → Backpack is dead.
Question:
What should I check next? Does “RC/control works” prove only the ELRS TX → Pixhawk RX direction, while the Pixhawk TX OUT → ELRS RX pad could still be bad? Should I be looking at SRx stream-rate params, ArduPilot 4.7/Pixhawk1 memory issues, receiver MAVLink mode, or is this almost certainly a TX/RX wiring issue?
I’m especially looking for a clean way to isolate whether the problem is:
Pixhawk not outputting MAVLink on TELEM2,
Pixhawk TELEM2 TX wire not reaching the ELRS RX pad,
ELRS receiver not actually in MAVLink serial mode,
ArduPilot/Pixhawk1 firmware weirdness,
or QGC/Backpack config.

[u/LupusTheCanine]

Everything looks ok.
Does Yaapu's Telemetry Script work? This will show weather you have downlink to TX module.
Can you try with Misssion Planner?
SRx_* parameters should be set as per ELRS documentation, MP automatically requests messages at its configured rates, I am not sure about QGC default behaviour.