I am trying to use a SIM900A based GSM modem. Whenever I am making the GND of module and arduino uno common, the GSM modem doesn't work. I tried the GSM modem with some other board and its perfectly working. What can be the issue with Arduino Uno's UART. I tried softwareserial (pin2 , pin3) as well as on default Tx, Rx of arduino, but still not working.