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Re: Using screen to talk to modems via AT commands

From: Tim Harvey
Subject: Re: Using screen to talk to modems via AT commands
Date: Fri, 21 Apr 2017 14:43:13 -0700

On Tue, Apr 18, 2017 at 1:12 AM, Amadeusz Sławiński <address@hidden> wrote:
> On Mon, 17 Apr 2017 08:00:01 -0700
> Tim Harvey <address@hidden> wrote:
>> Greetings,
>> I've often asked users I'm helping with modem support on Linux to use
>> GNU screen to issue AT
>> commands to modems to get their details (ie ATI), however I've noticed
>> this doesn't work for me anymore on perhaps newer modems and I'm
>> guessing its related to a missing stty setting relative to CRLF.
>> So the following works from bootup on an MC8790 for example:
>> # echo -e "ATI\r\n" > /dev/ttyUSB3
>> # cat /dev/ttyUSB3
>> ATI
>> Manufacturer: Sierra Wireless, Inc.
>> Model: MC8790
>> Revision: K1_0_2_8AP C:/WS/FW/K1_0_2_8AP/MSM6290/SRC 2008/09/17 14:00:22
>> IMEI: 352974021856935
>> IMEI SV: 3
>> FSN: D661380183210
>> 3GPP Release 6
>> OK
>> ^C
>> However if I use 'screen /dev/ttyUSB3 57600,cs8' I find that AT
>> commands are never recognized - I get no response from the modem as if
>> it didn't catch the right line termination.
>> Any ideas as to what's going on here and how I could use screen and/or
>> stty to resolve it or what the proper stty settings are to avoid this.
>> Regards,
>> Tim
> Hey,
> what version of screen do you use and which distribution?
> if it worked before, first thing which would help is narrowing down 
> working/not working version
> ( )
> And then perhaps using git bisect to narrow it down to specific change
> (use screen-v4 branch for that 
> )
> Cheers,
> Amadeusz


It seems to occur on every version of screen I've used including
4.03.01, 4.05.01, and master from the git repo however I've found that
screen does work fine on an x86 PC I have running Ubuntu 16.04 with
the Linux 4.4 kernel yet not with Ubuntu 16.04 with the Linux 4.4
kernel on armhf. So this seems to be an architecture thing. It used to
work and I can't figure out at what point it stopped working. Perhaps
its a flow control thing?



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