Interfacing SIM900A GSM Modem with TM4C123G TI microcontroller
The correct connections are: 3VR on GSM modem to TX port on launchpad, 3VT on GSM modem to RX port on launchpad and GND to GND. External Power supply too. you can test it with connecting the GSM modem to TTL to USB adapter and open putty and write "AT", if it replies "OK", its working.