Downlink command from Node-RED → ChirpStack → LoRa32u4 II not working

1

Hello everyone,

I am using a LoRa32u4 II node connected to ChirpStack with OTAA. Uplink is working correctly, and I can see the payloads in ChirpStack.

Now, I want to control a water pump through a downlink command:

  • The command will be sent from Node-RED to ChirpStack (via the MQTT integration).
  • Then ChirpStack should forward it to the device.
  • On the device, receiving 01 should turn ON the pump, and 00 should turn it OFF.

What I did:

  1. From Node-RED I publish a downlink payload to the ChirpStack MQTT topic.
  2. I also tested enqueuing a downlink manually from the ChirpStack web interface (FPort = 1, HEX payload 01).
  3. In my LMIC code, I check for downlinks in EV_TXCOMPLETE:

Screenshot 2025-08-18 130900
Screenshot 2025-08-18 1309001632×519 26.3 KB

The issue:

  • The uplink is fine.
  • But the downlink never reaches the device (LMIC.dataLen is always 0).
  • I cannot confirm whether ChirpStack is really sending the downlink to the gateway after Node-RED publishes it.

:point_right: My questions:

  • Is there a specific configuration needed in ChirpStack to forward downlinks coming from Node-RED?
  • How can I check in ChirpStack logs if the downlink is correctly queued and transmitted to the gateway?
  • Do I need to adjust something in LMIC regarding RX1/RX2 windows for EU868?

Thanks a lot for your help :pray: