RAK3172 downlink confirmation behaviour issues


We have been testing the RAK3172 module with the latest RUI3 firmware as a LoRaWAN modem. The module is connected via UART interface to an RP2040.
Everything is working fine with the only exception being confirmed downlinks.

In order to respond to application commands, the RP2040 is periodically (very short delay) checking if any downlinks have arrived with the AT+RECV command and immediately issuing an AT+SEND command in response. No matter how fast we send the response though, the modem acknowledges the downlink automatically with an uplink on fport 0 and, only after that, it sends the actual response.

Here is a picture of the Chirpstack events of the exchange:

Is there a way to change this behaviour so that we can acknowledge a confirmed downlink with the following device initiated AT+SEND command instead? Or to just modify the payload and fport of the automatically issued response? So far I have not been able to find any info regarding this.

Best Regards.

Welcome to the forum @dancho

On a confirmed downlink, you cannot send a manual ACK. The answer to the confirmed downlink with ACK is handled by the LoRaWAN stack already. It is part of the protocol and not handled by the application.