[xobs]

ModemManager. You need to set the variable ENV{ID_MM_PORT_IGNORE}=“1” I. A udev rule.

Standard usb serial ports show up as ttyACM#, whereas nonstandard ports that require a driver like ftdi show up as ttyUSB#. Modems tend to be standard usb devices, so ModemManager by default scans all serial ports as if they were modems. This involves sending some AT commands to them to try and identify them.

Software implementations of serial devices tend to follow the standard, so they show up as ttyACM#.

[foresto]

Thanks for the tip. Unfortunately, it doesn't seem to be the cause of my one-way serial port issue. Adding the udev environment variable makes no difference, nor does stopping the ModemManager service.