Endless data loop from dev/ttyUSB while sending AT commands

Question

I have problem sending AT commands to my USB modem. At first I tried php-serial class, but when I was reading data, I got endless loop with strange data. Then I tried to debug problem and opened putty terminal, made command

cat /dev/ttyUSB0

and then sent command echo "AT" > /dev/ttyUSB0 after that I always got never ending loop from cat /dev/ttyUSB0 like this:

AT
AT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^JAT^

and this loop never ends. On Windows all works just fine. I can connect to USB modem via virtual COM port and send command / get answer.

Question is why can't I just receive "OK" message from console, but instead of this I got this data loop?

Tried different huawei USB modems, and different stty settings and also default settings for huawei modems:

Baud rate: 9600 / 115200
Parity: none
Data bits = 8
Stop bits = 1
Flow control = none


test dev # uname -a
Linux test 3.5.7-gentoo #3 SMP Sun Feb 17 04:58:22 EET 2013 x86_64 AMD Athlon(tm) 64 X2 Dual Core Processor 5200+ AuthenticAMD GNU/Linux

compiled newer kernel: 3.6.11-gentoo and now when sending command:

echo "AT" > /dev/ttyUSB0

got result like:

OK
AT
OK
AT
OK
AT

it never ended, I terminated cat command, then sent other ( make call command ):

echo "ATDT 27789388;" > /dev/ttyUSB0

and got answer from cat without loop.

systest ~ # cat /dev/ttyUSB0
ATDT 27789388;


OK

so why tty terminal is acting so strange? maybe I don't know something about sending/reading data on serial ports?

if it can help then output of stty -a -F /dev/ttyUSB0 command is:

speed 9600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke

Answer 1

Doing AT command communication from the command line with just plain shell operations is rather unreliable. I suggest that you try to use the program atinout which is specifically written to issue AT command from the command line:

$ echo AT | atinout - /dev/ttyUSB0 -
AT
OK
$

Answer 2

Use "ATE0" Before

echo "ATE0" > /dev/ttyUSB0

Answer 3

Check to see if modem-manager is running -- and kill it if it is.

If it thinks that the device is a modem, it will step in and try to dial the modem also. This fixed as similar problem for me recently.

Answer 4

Definitely look like your terminal setup is messed up. Check and play with these stty settings:

echo echoe echok -echonl -echoprt echoctl echoke

EDIT: The reason you got the AT - OK loop, is that you have enabled local terminal echo in both your terminal and on the modem side in the AT interpreter. To turn off echo in the AT interpreter, issue: ATE0 (that's a zero) as your first command.

Answer 5

The command:

stty -echo -F /dev/ttyUSB3

worked for me