I'm working on a IMX6 based board, and I need some advice to handle my modem.

I've successfully configure the modem, and mounted the ppp. The modem is working fine and everything's great, but now I want to send him some AT command while running.

Here is the content of my option file :

maxfail 0
connect "/usr/sbin/chat -v -f /etc/ppp/connect"

The modem is on /dev/ttyACM0, and of course, when ppp is mounted, the file is locked. I have not put 'lock' in the /etc/ppp/option file. I have tried to put nolock in it too, but the file is still locked :(

Is there a way to send command to the modem without unmount the ppp ? How ? Thanks.


It is not generally possible to send AT commands to a modem while a call is in progress. This applies to all AT-compatible modems, "regular" dial-up modems and mobile phone modems alike, and it has nothing to do with PPP.

By the way, a point of terminology: PPP sessions are not mounted. Mouting and umounting are terms that apply to filesystems.

There are two ways that commands can be sent to a modem while a call is in progress:

  • Using the +++ escape sequence. You must sent the three characters +++ surrounded by one second of idle (no transmission) before and after. The modem will return to command mode. The call is suspended following execution of the escape sequence (which means that your PPP session will probably time out and break if you are not fast). Use ATO to return to the call. Note that the +++ escape sequence is often disabled (unavailable), and sometimes even if it is enabled it will drop the call unstead of suspending it. The +++ escape sequence is a very old standard is is not commonly used today.
  • If the modem has more than one serial port connecting it to the computer, then you can send commands on one serial port while a call is in progress on the other one. Traditional dial-up or ISDN modems never have this feature, but I believe modern mobile phone modems might.
  • Thanks a lot ! Very nice answer :) BTW, by searching solutions, I've found libmm-glib, who can be really helpful to manage modems on linux. – MokaT Aug 25 '15 at 15:01

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