AT Commands, GSM AT command set
September 11, 2021 By Nikhil Agnihotri
AT commands are used to control MODEMs. AT is the abbreviation for Attention. These commands come from Hayes commands that were used by the Hayes smart modems. The Hayes commands started with AT to indicate the attention from the MODEM. The dial up and wireless MODEMs (devices that involve machine to machine communication) need AT commands to interact with a computer. These include the Hayes command set as a subset, along with other extended AT commands.
AT commands with a GSM/GPRS MODEM or mobile phone can be used to access following information and services:
1. Information and configuration pertaining to mobile device or MODEM and SIM card.
2. SMS services.
3. MMS services.
4. Fax services.
5. Data and Voice link over mobile network.
The Hayes subset commands are called the basic commands and the commands specific to a GSM network are called extended AT commands.
Types of AT Commands:
There are four types of AT commands:
Image Showing Classification of AT Commands
Fig. 1: Image Showing Classification of AT Commands
1) Test commands – used to check whether a command is supported or not by the MODEM.
SYNTAX: AT<command name>=?
For example: ATD=?
2) Read command – used to get mobile phone or MODEM settings for an operation.
SYNTAX: AT<command name>?
For example: AT+CBC?
3) Set commands – used to modify mobile phone or MODEM settings for an operation.
SYNTAX: AT<command name>=value1, value2, …, valueN
Some values in set commands can be optional.
For example: AT+CSCA=”+9876543210”, 120
4) Execution commands – used to carry out an operation.
SYNTAX: AT<command name>=parameter1, parameter2, …, parameterN
The read commands are not available to get value of last parameter assigned in execution commands because parameters of execution commands are not stored.
For example: AT+CMSS=1,”+ 9876543210”, 120
Explanation of commonly used AT commands:
1) AT – This command is used to check communication between the module and the computer.
The command returns a result code OK if the computer (serial port) and module are connected properly. If any of module or SIM is not working, it would return a result code ERROR.
2) +CMGF – This command is used to set the SMS mode. Either text or PDU mode can be selected by assigning 1 or 0 in the command.
0: for PDU mode
1: for text mode
The text mode of SMS is easier to operate but it allows limited features of SMS. The PDU (protocol data unit) allows more access to SMS services but the operator requires bit level knowledge of TPDUs. The headers and body of SMS are accessed in hex format in PDU mode so it allows availing more features.
3) +CMGW – This command is used to store message in the SIM.
SYNTAX: AT+CMGW=” Phone number”> Message to be stored Ctrl+z
As one types AT+CMGW and phone number, ‘>’ sign appears on next line where one can type the message. Multiple line messages can be typed in this case. This is why the message is terminated by providing a ‘Ctrl+z’ combination. As Ctrl+z is pressed, the following information response is displayed on the screen.
+CMGW: Number on which message has been stored
4) +CMGS – This command is used to send a SMS message to a phone number.
SYNTAX: AT+CMGS= serial number of message to be send.
As the command AT+CMGS and serial number of message are entered, SMS is sent to the particular SIM.
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