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I installed the debian wheezy and use the usb dialup modem to connect the internet, the modem port is "/dev/ttyACM0" and it was woking, but snice few days this problem araised.
During connection the system changing the usb modem id between "/dev/ttyACM0" and "/dev/ttyACM1" without any reason even i ddin't attach any usb device. this is the /var/log/messages:
Jan 28 13:30:26 example kernel: [ 6.579461] input: HDA Intel Headphone as /devices/pci0000:00/0000:00:1b.0/sound/card0/input7
Jan 28 13:30:26 example kernel: [ 6.598449] cdc_acm 4-2:1.0: ttyACM0: USB ACM device
Jan 28 13:30:26 example kernel: [ 6.600125] usbcore: registered new interface driver cdc_acm
Jan 28 13:30:26 example kernel: [ 6.600128] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
Jan 28 13:57:09 example pppd: Modem hangup
Jan 28 13:57:09 example pppd: Connect time 23.9 minutes.
Jan 28 13:57:09 example pppd: Sent 540090 bytes, received 4299636 bytes.
Jan 28 13:57:09 example pppd: Connection terminated.
Jan 28 13:57:09 example kernel: [ 1615.888052] usb 4-2: USB disconnect, device number 2
Jan 28 13:57:10 example kernel: [ 1616.128017] usb 4-2: new full speed USB device number 3 using uhci_hcd
Jan 28 13:57:10 example kernel: [ 1616.813357] usb 4-2: New USB device found, idVendor=0572, idProduct=1329
Jan 28 13:57:10 example kernel: [ 1616.813362] usb 4-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
Jan 28 13:57:10 example kernel: [ 1616.813365] usb 4-2: Product: USB Modem
Jan 28 13:57:10 example kernel: [ 1616.813368] usb 4-2: Manufacturer: Conexant
Jan 28 13:57:10 example kernel: [ 1616.813370] usb 4-2: SerialNumber: 24680246
Jan 28 13:57:10 example kernel: [ 1616.816419] cdc_acm 4-2:1.0: ttyACM1: USB ACM device
Jan 28 13:57:10 example mtp-probe: checking bus 4, device 3: "/sys/devices/pci0000:00/0000:00:1d.2/usb4/4-2
This is lsusb:
Bus 004 Device 003: ID 0572:1329 Conexant Systems (Rockwell), Inc.
I run the above command and created the link, but ttyACM1 disappear after reboot, when i disconnect the usb modem cable from the port the /dev/ttyACM0 disappearing as well and after reconnect it come again. so i should make the links after every system boot.
/dev is mounted on some temporary system, and vanishes every time. Udev makes /dev/acm0 when it existys, and removes it when it doesn't exist. You don't really want your pc to talk to the modem while it's not there, do you? find ACM0 (in /lib/udev/ruled.d or somewhere your distro felt like putting it) and ls -l it, it will have a major and a minor number
<type>perms user group ACM0 x y
where y is the minor number. Add 1 to the minor and you can add ACM1