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.
5) ATD - This command is used to dial or call a number.
SYNTAX: ATD<Phone number>;(Enter)
6) ATA - This command is used to answer a call. An incoming call is indicated by a message ‘RING’ which is repeated for every ring of the call. When the call ends ‘NO CARRIER’ is displayed on the screen.
As ATA followed by enter key is pressed, incoming call is answered.
7) ATH - This command is used to disconnect remote user link with the GSM module.
SYNTAX: ATH (Enter)