THE SERIAL PORT on my Fedora 15 install, mysteriously refused to be accessed. I discovered that when inserting a USB-to-Serial device, GNU screen would refuse to access the created device /dev/ttyUSB0.
» screen /dev/ttyUSB0 115200
[screen is terminating]
Since I could use screen for serial access as root, and because the newly installed Fedora did have some hiccups in adding my user (the /home/monzool directory already existed from a previous Ubuntu install), I first checked group permissions, but they seemed fine for this situation.
» ll /dev/ttyUSB0
crw-rw----. 1 root dialout 188, 0 Jun 6 08:27 /dev/ttyUSB0
» groups
monzool tty wheel uucp dialout tcpdump screen vboxusers
Screen didn’t offer much indication of the problem, but using strace I could see that some of the last things checked for permissions where /var/run/screen. I then removed that directory and recreated the directory setup by starting screen with sudo.
» ll /var/run/screen
drwxrwxr-x. 4 root root 80 Jun 6 08:27 screen
» rm -rf /var/run/screen
» sudo screen /dev/ttyUSB0
» ll /var/run/screen
drwxrwxr-x. 4 root screen 80 Jun 6 08:47 screen
This helped nothing! :-(
I then tried minicom, which was more informative about the problem
» minicom
minicom: cannot open /dev/ttyUSB0: Device or resource busy
This would mean that something else had hijacked the port. A quick check confirmed that something called gpsd was using the port.
» lsof /dev/ttyUSB0
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
gpsd 883 nobody 8u CHR 188,0 0t0 11408 /dev/ttyUSB0
» ps ax | grep gpsd
883 ? S
Now gpsd is for handling GPS devices, but it made no sense to trigger this daemon for a simple USB-to-Serial adapter.
Knowing what was causing the hazzle, I found this bug rapport https://bugzilla.redhat.com/show_bug.cgi?id=663124. In it, it is proposed to set USBAUTO=no in /etc/sysconfig/gpsd.
» echo "USBAUTO=no" >> /etc/sysconfig/gpsd
And sure enough, this fixed the problem. The USB-to-Serial adapter could now be accessed by any serial terminal.