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I am trying to use an USB-Serial converter with a program (a deamon) that is launched at startup on a embedded board (Odroid-XU). The deamon does not work since it fails to open the serial port /dev/ttyUSB0 reporting that the port is already in use (Device or resource busy).

Thus my first idea was that something uses the serial port shortly after the boot. BUT when I unplug and plug back the USB-Serial adapter, I have the same issue: I cannot use it for a few seconds. I checked with an oscilloscope the output of the TX port and it appears that it receives commands after being plugged or after the boot.

So what is happening here ? And what would be the solution to avoid that ?

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I solved my problem using sudo lsof /dev/tty* to find out what process was using the serial port. Then simply removing modemmanager from the boot sequence solved my problem. BTW, I found the answer I needed a bit too late. I should spend more time on superuser.

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