MC 8705, Which USB port to use for pppd.

I am using Sierra wireless Airprime MC 8705 module.
After connecting this module, i am getting 5 serial USB interface, output is given below :

root@voyage:~# ls /dev/ttyUSB*
/dev/ttyUSB0 /dev/ttyUSB1 /dev/ttyUSB2 /dev/ttyUSB3 /dev/ttyUSB4

  • i wanted to know how to identify which interface to use for AT command for PPPD?
    (after trying on all interface, i got to know for PPPD we need to use ttyUSB3 interface for doing AT commands).

  • Why we have 5 ttyUSB interface. What is the use for each usb interface ?

  • Is there a way to identify the correct ttyUSB interface through sysfs filesystem using USB endpoints ?

OS : Linux Debian 7

Thanks and Regards,


The ports are for the following functions

  • DM (Diagnostic monitoring).
  • CnS
  • AT
  • NMEA
  • Modem

There is no way to determine what each of them are from within the system other than to know when you plug a unit in they will always enumerate in the same order so you can determine what port is what even if there are already some /dev/ttyUSB ports on your system), it can vary on a unit basis depending on what is enabled.

There are a couple of ways to determine what port is what during system design. Through AT commands and to determine the full at parser (AT) and modem ports send the ATI command to them, the modem port will return with an APP1 response where the full AT parser will come back with unit type, firmware information, etc.

If you want to see the config it will bot into and what other configs there are send the below commands to the unit

at!udusbcomp?      //This shows the current config
at!udusbcomp=?    //This shows what is supported on the current unit. The order they are shown in is the order they will enumate in on the system.