nRF9160 mqtt server published message is split in chunks by 120 bytes by UART

[Spartak]

Hello, Guys.

I just wondered why max UART exchange when we receive mqtt buffer that is let's say 1707 bytes is splited bu uart in chunks max of 120 bytes.

Is there some UART transfer limitation in serial LTE modem?

[Didrik Rokhaug]

Hi,

Which SDK version are you using?

Is the problem that the device is sending 120 byte chunks over the air, or that when you receive a message, it is transmitted in 120 byte chunks on the UART?

Do you have any logs?

(The SLM sends logs over RTT by default. You can enable more logging by setting CONFIG_SLM_LOG_LEVEL_DBG=y in your prj.conf)

Best regards,

Didrik

[Spartak]

SDK - 2.1.0 

It is transmitted in 120 byte chunks on the UART.

I use external MCU to control it so no logs output.

[Didrik Rokhaug]

Like I said in my previous reply, the SLM sends logs over RTT by default, which goes over the debugger interface. Are you able to connect a debugger to the nRF9160, and get the RTT logs?