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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, 5 May 2017 18:05:57 -0700

On Mon, Apr 17, 2017 at 8:00 AM, 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
> 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.

Now I feel rather silly. This was due to me using screen within a
screen session in which case screen starts translating CR to LF and LF
to CR. Because modems use CR as the line termination character the
lines were never terminated because screen within a screen session
would send a LF every time I hit CR.


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