There are four types of AT command operations:
Test operation. A test operation is used to check whether a certain AT command is supported by the GSM/GPRS modem or mobile phone.
Set operation. A set operation is used to change the settings used by the GSM/GPRS modem or mobile phone for certain tasks.
Read operation. A read operation is used to retrieve the current settings used by the GSM/GPRS modem or mobile phone for certain tasks.
Execution operation. An execution operation is used to perform an action or retrieve information/status about the GSM/GPRS modem or mobile phone.
The command syntax for performing an operation will be described in detail in the following sections.
Test Command -- Checks Whether a Certain AT Command is Supported
A test operation is used to check whether a certain AT command is supported by the GSM/GPRS modem or mobile phone. All extended AT commands support the test operation. The syntax is:
command=?
where command is an AT command. When an AT command is used in the above syntax to perform a test operation, it is called a test command.
Here is an example. The AT command +CGMI (command name in text: Request Manufacturer Identification) is used to get the manufacturer name of the GSM/GPRS modem or mobile phone. To test whether +CGMI is supported, you can make use of the test command "+CGMI=?". The complete command line that should be entered is:
AT+CGMI=?
If the GSM/GPRS modem or mobile phone supports the AT command +CGMI, the result code "OK" will be returned, like this:
AT+CGMI=?
OK
If the GSM/GPRS modem or mobile phone does not support the AT command +CGMI, the result code "ERROR" will be returned, like this:
AT+CGMI=?
ERROR
In the above example, the AT command +CGMI does not have any parameters. If the AT command to be tested has parameter(s), the parameter value(s) supported by the GSM/GPRS modem or mobile phone may be printed additionally. Below is an example that illustrates the format of the response. +COMMAND1 is a fictitious AT command that has four parameters.
AT+COMMAND1=?
+COMMAND1: (0,1),(0-10),(0,1,5-10),("GSM","UCS2")
OK
The supported values of each of the four parameters are enclosed in parentheses. Commas are used to delimit the parentheses and the values inside parentheses. A hyphen is used to indicate a range of values. The values inside parentheses can be of the string type.
In the above example, the response of the test command "+COMMAND1=?" provides us the following information:
(0,1). The first parameter accepts either 0 or 1.
(0-10). The second parameter accepts any integer between 0 and 10.
(0,1,5-10). The third parameter accepts 0, 1 or any integer between 5 and 10.
("GSM","UCS2"). The fourth parameter accepts either the string "GSM" or "UCS2".
To a few AT commands, the test operation does not return the parameter values supported. Instead, it returns the values that are allowed to appear in the information response of the AT command. An example is the +CBC AT command (command name in text: Battery Charge). The +CBC command is used to retrieve the connection status and charge level of the battery of the mobile device. Two values are returned in the information response of the +CBC AT command. The format is:
+CBC: connection_status,charge_level
For example, if the battery is placed in the mobile device with no charger connected and the charge level is 80%, the result of the execution of the +CBC AT command will be:
AT+CBC
+CBC: 0,80
OK
If you run the test command "+CBC=?", all the supported values that are allowed to appear in the connection status field and charge level field will be provided. With my Nokia 6021, the result is:
AT+CBC=?
+CBC: (0,1),(0-100)
OK
"(0,1)" means the connection status field in the information response of the +CBC AT command can contain either 0 or 1, while "(0-100)" means the charge level field can contain any integer between 0 and 100.
Set Command -- Changes the Settings Used for Certain Tasks
A set operation changes the settings used by the GSM/GPRS modem or mobile phone for certain tasks. The syntax is:
command=value1,value2,...valueN
where command is an AT command and value1 to valueN are the values you want to set. When an AT command is used in the above syntax to perform a set operation, it is called a set command.
Here is an example. The AT command +CSCA (command name in text: Service Centre Address) is used to set the SMSC (SMS center) address for sending SMS messages. It takes two parameters that specify the SMSC address and type of address. To set the SMSC address to +85291234567, enter the following command line in a terminal program such as MS Windows' HyperTerminal:
AT+CSCA="+85291234567",145
If the set command runs successfully, the result code "OK" will be returned:
AT+CSCA="+85291234567",145
OK
Some AT commands have optional parameters. You can choose not to assign values to them. For example, the second parameter of the +CSCA AT command is optional. If no value is assigned to the second parameter, the GSM/GPRS modem or mobile phone will use the default parameter value, which is 145 if the SMSC address starts with "+" (the plus character). Hence, this command line:
AT+CSCA="+85291234567"
is equivalent to:
AT+CSCA="+85291234567",145
Typically the values you specified with set commands are placed in volatile memory. If the GSM/GPRS modem or mobile phone is switched off or rebooted, the values you specified with set commands will be gone. When the GSM/GPRS modem or mobile phone is powered on again, all settings are back to the defaults.
For some commonly used settings, there are AT commands for saving/restoring the settings to/from non-volatile memory. For example, the AT commands +CSAS (command name in text: Save Settings) and +CRES (command name in text: Restore Settings) can be used to save and restore settings related to SMS messaging such as the SMS center address.
Read Command -- Retrieves the Current Settings Used for Certain Tasks
A read operation retrieves the current settings used by the GSM/GPRS modem or mobile phone for certain tasks. The syntax is:
command?
where command is an AT command. When an AT command is used in the above syntax to perform a read operation, it is called a read command. The read operation is supported by all AT commands that are capable of the set operation.
Here is an example that illustrates how to use a read command. The AT command +CSCA (command name in text: Service Centre Address) is used to set the SMSC (SMS center) address for sending SMS messages. It takes two parameters that specify the SMSC address and type of address. Suppose you set the SMSC address to +85291234567 in Microsoft HyperTerminal, like this:
AT+CSCA="+85291234567",145
OK
After that, if you enter the read command "+CSCA?", the GSM/GPRS modem or mobile phone will return the SMSC address and type of address that you set in the previous step:
AT+CSCA?
+CSCA: "+85291234567",145
OK
Execution Command -- Performs an Action or Retrieve Information/Status about the GSM/GPRS Modem or Mobile Phone
An execution operation is used to perform an action (for example, send or read an SMS message) or retrieve information/status about the GSM/GPRS modem or mobile phone (for example, retrieve the current battery charge level, battery charging status or radio signal strength of the mobile network). The syntax is:
command=value1,value2,...valueN
where command is an AT command and value1 to valueN are the values to assign to the AT command. If the AT command does not have any parameters, the part "=value1,value2,...valueN" should be omitted. When an AT command is used in the above syntax to perform an execution operation, it is called an execution command.
Here is an example illustrating the use of an execution command. The AT command +CMSS (command name in text: Send Message from Storage) can be used to perform an execution operation to send an SMS message stored in message storage. It has three parameters. They specify the index of the memory location that stores the SMS message, the destination phone number and the type of the phone number respectively. To send the SMS message at index 1 to the phone number +85291234567, the following command line can be used:
AT+CMSS=1,"+85291234567",145
Some AT commands have optional parameters. You can choose not to assign values to them. For example, the third parameter of the +CMSS AT command is optional. If no value is assigned to the third parameter, the GSM/GPRS modem or mobile phone will use the default parameter value, which is 145 if the destination phone number starts with "+" (the plus character). Hence, this command line:
AT+CMSS=1,"+85291234567"
is equivalent to:
AT+CMSS=1,"+85291234567",145
Unlike set commands, execution commands do not store the parameter values assigned to them. So, no read command is available for retrieving the last parameter values assigned to an execution command. For example, if you send the command line "AT+CMSS?" to your GSM/GPRS modem or mobile phone, the ERROR result code will be returned:
AT+CMSS?
ERROR
