GSM SIM 900A coming with the interface between TTL logic to RS-232 through chip MAX-232. The RS-232 interface on GSM/GPRS Modem gives the features to connect PC as well as Microcontroller. This modem is works on the frequencies 900/1800 MHz and the baud rate is configurable from 9600-115200 through AT command. The GSM/GPRS Modem is having internal TCP/IP stack to enable us to connect with internet via GPRS. It is suitable for SMS, Voice as well as DATA transfer application in M2M interface. The onboard Regulated power supply allows us to connect wide range unregulated power supply. Using this modem, we can make audio calls, SMS, Read SMS, attend the incoming calls and internet etc. through simple AT commands.

Figure 3: GSM module SIM900A

AT COMMANDS OF SIM900A

To check the modem:

AT

OK

To change SMS sending Mode:

AT+CMGF=1

To send new SMS:

AT+CMGS=”MOBILE NO.”

<MESSAGE

{CTRL+Z}

To make a voice call:

ATD8567890365;

To redial last no:

ATDL

To receive incoming call:

ATA

To hang up or disconnect a call:

ATH