How to send AT commands to device

AT commands (“attention commands” formally, the Hayes command set), are used to communicate directly with a modem device and configure it.

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 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 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.

Use echo -e so as to be able to escape characters such as “.

To send AT commands 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/ttyUSB0
Connection: 115200 @ 8-N-1
Line end: CR

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)

Send: AT
OK

Send: AT^SETPORT=?
Recieve: 1:MODEM
Recieve: 2:PCUI 
Recieve: 3:DIAG
Recieve: 4:PCSC
Recieve: 5:GPS
Recieve: 6:GPS CONTROL
Recieve: 7:NDIS
Recieve: A:BLUE TOOTH
Recieve: B:FINGER PRINT
Recieve: D:MMS
Recieve: E:PC VOICE
Recieve: A1:CDROM
Recieve: A2:SD
Recieve: OK

Send: AT^SETPORT?
Recieve: A1,A2;1,2,3,A1,A2
Recieve: OK

Send: AT^SETPORT="A1;2,7,A2"
Recieve: OK

Send: AT^SETPORT?
Recieve: A1;2,7,A2
Recieve: 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 PC interface! (2:PCUI in this example) 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"

Format:

AT+QCFG="nwscanmode"[,<scanmode>[,<effect>]]

Parameters：

<scanmode> Number format, network search mode
0 AUTO
1 GSM only
2 UMTS only
3 LTE only

<effect> Number format, when to take effect
0 Take effect after UE reboots
1 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

• Chromium project page on debugging cellular modems
• Wikipedia article on AT commands and their history