ncs-serial-mode dtr/ri operation

RE: ncs-serial-mode dtr/ri operation

Vidar Berg — Fri, 30 Jan 2026 06:28:55 GMT

I’m glad to hear you found the problem. Thank you for reporting back on what the issue was.

RE: ncs-serial-mode dtr/ri operation

thipp — Thu, 29 Jan 2026 18:19:32 GMT

Thank you.
I found my issue. I had somehow created a duplicate overlay-external-mcu.overlay file in the "apps" folder and my build was pointing to it instead of the correct overlay file.

RE: ncs-serial-mode dtr/ri operation

Vidar Berg — Thu, 29 Jan 2026 14:51:57 GMT

Hello,

You can confirm what is being applied to the build by inspecting the generated zephyr.dts file for your modem app:

Are you using the nrf9151dk as your build target, or have you defined your own board?

RE: ncs-serial-mode dtr/ri operation

thipp — Thu, 29 Jan 2026 14:15:42 GMT

FYI, my yellow hightlighting masked the fact when SM_DTR_GPIOS is not properly defined, the highlighted text is grayed out.

RE: ncs-serial-mode dtr/ri operation

thipp — Thu, 29 Jan 2026 14:11:03 GMT

Here is my investigation on whether or not the pins are properly configured.
If I go to my root build_mcu_ext configuration, it looks like the sm_ctrl.pin.c SM_DTR_GPIOS is properly defined.

But if I select build_mcu_ext/ncs-serial-modem configuration, SM_DTR_GPIOS is not properly defined.

If I do a search for ncs_sm_uart, it looks like my build_evk is setup for a dtr_uart, but my build_mcu_ext is not

I am following all the documentation for using the overlay-external-mcu.overlay, what should I expect to see?

RE: ncs-serial-mode dtr/ri operation

thipp — Thu, 29 Jan 2026 13:38:42 GMT

Vidar,

This is a custom board with external mcu. Both the mcu and modem reference the same GND. The DTR/RING signals are wired per the documented interface setup and the voltage levels match (1.8V). I will try and enable the CONFIG_DTR_UART_LOG_LEVEL_DBG to see what it provides. Is there a way to review generated output files to confirm the DTR and RING pins are configured properly?

RE: ncs-serial-mode dtr/ri operation

Vidar Berg — Thu, 29 Jan 2026 12:38:16 GMT

Hello,

I forwarded your observations to the developers, and they suggested that you try the following:

Check the wiring so that the DTR and RI are not mixed up. DTR should go to DTR and RI should go to RI.
Check that the voltage levels in gpio's match (this is by far the most common reason for failures).
https://docs.nordicsemi.com/bundle/addon-serial_modem-v0.3.0/page/uart_configuration.html actually seems to miss the gpio GND <-> gpio GND, so that should be added
Check the device tree that they have nothing else in UART or DTR/RI pins. They are using different pins than with nRf9151DK, so I take it that this is not a DK. If it is then they should try the default pins.
Check the DTR assertion from RTT logs with: CONFIG_DTR_UART_LOG_LEVEL_DBG

Best regards,

Vidar