AT commands (“attention commands” formally, the Hayes command set), are used to communicate directly with a modem device and configure it.
From OpenWrt
To send AT commands directly from OpenWrt, you can simply use echo
to send them to the right device. However, you will not get any errors, confirmation or any other answer from the modem.
A better solution is using a proper terminal application like picocom
, which can both send commands and print the modem's answers. There are other alternatives available like socat
.
Once the appropriate serial driver is loaded (typically - kmod-usb-serial-option
), the modem will expose a number of /dev/ttyUSBx
interfaces. Usually only one or two of them will respond to AT commands.
As an example, popular Quectel EP06 LTE modem will create 4 serial devices, /dev/ttyUSB0
through to /dev/ttyUSB3
.
To find the serial devices added to the system, try looking through logread
and/or dmesg
output. Something like the following may help:
dmesg | grep -A 1 -B 12 ttyUSB
For some modems ttyACM
devices will be created instead of ttyUSB
.
picocom
picocom
is a simple terminal program that is installed in a standard way with opkg install picocom
or through the web interface.
Sample command line:
picocom /dev/ttyUSB2
Adding --q
on the command line will suppress extra output with some help and other information.
socat
socat
will open a prompt where you can enter AT commands (see examples in the section below).
Here is a command line example:
socat - /dev/ttyUSB2,crnl
In this example socat
will send a carriage return (cr) and a new line (nl) after each command.
To quit socat
, use ctrl+C
.
echo
See examples below. echo -e
is used to send escaped characters such as quotation marks.
Alternative solution - use echo
with socat
: echo -e ATI | socat - /dev/ttyUSB2,crnl
From a computer
To send ATcommands to a LTE modem, you need to first connect the device/modem to the computer, most likely using an adapter (built-in modem slots are very rare these days) and access it with a COM terminal.
If you are not familiar with using COM terminals, you might want to use a graphical tool like: CuteCom
or minicom
. Installation of these are beyond the scope of this page.
These settings should work fine:
Device: /dev/ttyUSB0Connection: 115200 @ 8-N-1Line end: CR
Examples of AT commands
To test things are working, you can issue a standard ATI
command which should return basic information such as brand, model and firmware revision.
AT+CSQ
can be used to get signal strength. The values returned are the RSSI (received signal strength indication, higher is better) and BER (bit error rate, lower is better)
Huawei E392
Send: ATOKSend: AT^SETPORT=?Recieve: 1:MODEMRecieve: 2:PCUI Recieve: 3:DIAGRecieve: 4:PCSCRecieve: 5:GPSRecieve: 6:GPS CONTROLRecieve: 7:NDISRecieve: A:BLUE TOOTHRecieve: B:FINGER PRINTRecieve: D:MMSRecieve: E:PC VOICERecieve: A1:CDROMRecieve: A2:SDRecieve: OKSend: AT^SETPORT?Recieve: A1,A2;1,2,3,A1,A2Recieve: OKSend: AT^SETPORT="A1;2,7,A2"Recieve: OKSend: AT^SETPORT?Recieve: A1;2,7,A2Recieve: OK
Explanations:
AT^SETPORT=? - Lists the Available interfaces and their numbers AT^SETPORT? - Show current configuration AT^SETPORT="A1;2,7 - Sets configuration.
Modem configuration is split into 2 parts: before ;
and after.
Once a modem is plugged-in, it should declare itself in first configuration (normally with at least: A1 - virtual CD drive with Drivers and application). If the drivers are installed, they will see the modem and issue a special command to switch to a “working” configuration - this is 2,7 interfaces in this example.
Warning: Never turn Off the AT Command interface! (“PCUI” in Huawei terms) You will lose the ability to access modem with terminal and change the configuration.
You can add more interfaces to be active i.e. SD card:
AT^SETPORT="A1,A2;2,7,A2"
If you get an ERROR, maybe the numerical mode is not sorted (16,2,7)→(2,7,16). If your device answers to set command with OK but AT^SETPORT?
doesn't show your desired settings, you can try using space in between numerical modes(2,7) and alphabetical modes(A2) like so:
AT^SETPORT="A1,A2;2,7,A2"
Or with multiple modes:
AT^SETPORT="A1,A2;2,7,A1,A2"
Quectel modems
How to force LTE connection
Format:
AT+QCFG="nwscanmode"[,<scanmode>[,<effect>]]
Parameters:
<scanmode> Number format, network search mode0 AUTO1 GSM only2 UMTS only3 LTE only<effect> Number format, when to take effect0 Take effect after UE reboots1 Take effect immediately
Examples:
Set modem to LTE only:
echo -e "AT+QCFG=\"nwscanmode\",3,1" > /dev/ttyUSB2
Set it back to “auto”:
echo -e "AT+QCFG=\"nwscanmode\",0,1" > /dev/ttyUSB2
Further references
Chromium project page on debugging cellular modems
Wikipedia article on AT commands and their history
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