Hi,

I am having problems receiving AT commands from a 4G modem from node-red with the serial node. I don't receive a response from the modem.

If I run "cat /dev/ttyUB2" in the terminal, I can see all the responses from the modem, so the messages I am sending from node-red work.

Could it be that node-red does not have priority to read /dev/ttyUB2?

SOLVED

ModemManager was taking control of the ttyUSB2 and ttyUSB3 ports that allow communication by AT commands.

I created the file "78-mm-quectel-port-types.rules" with rules to ignore those two ports and placed them in the "/usr/lib/udev/rules.d" folder and then rebooted the IOT2050.

I generated the rules file "78-mm-quectel-port-types_my_rules.rules" from "/usr/lib/udev/rules.d/77-mm-quectel-port-types.rules" because I am using the Quectel EC25 modem. I cleaned up all the rules I didn't need and put in new rules so that ModemManager ignores ports ttyUSB2 and ttyUSB3, and doesn't capture them.

I attached the file "78-mm-quectel-port-types_my_rules.rules.txt" but then it must be renamed without the ".txt" extension since otherwise I could not attach it.

78-mm-quectel-port-types_my_rules.rules:

ACTION!="add|change|move|bind", GOTO="mm_quectel_port_types_end_my_rules"
SUBSYSTEMS=="usb", ATTRS{idVendor}=="2c7c", GOTO="mm_quectel_port_types_my_rules"
GOTO="mm_quectel_port_types_end_my_rules"

LABEL="mm_quectel_port_types_my_rules"

SUBSYSTEMS=="usb", ATTRS{bInterfaceNumber}=="?*", ENV{.MM_USBIFNUM}="$attr{bInterfaceNumber}"

# Quectel EC25/EG25
#  ttyUSB2 (if #2): Se ignora para que node-red pueda leerlo
#  ttyUSB3 (if #3): Se ignora para que node-red pueda leerlo
ATTRS{idVendor}=="2c7c", ATTRS{idProduct}=="0125", ENV{.MM_USBIFNUM}=="02", ENV{ID_MM_PORT_IGNORE}="1"
ATTRS{idVendor}=="2c7c", ATTRS{idProduct}=="0125", ENV{.MM_USBIFNUM}=="03", ENV{ID_MM_PORT_IGNORE}="1"

LABEL="mm_quectel_port_types_end_my_rules"

I obtained this information from:
https://modemmanager.org/docs/modemmanager/port-and-device-detection/

I think this can also be useful for GPS or other devices as well. I saw that this problem was very common to communicate via serial port with Arduino and Raspberry PI.

Regards