SIM900 Arduino Shield AT Serial

sim900_at.ino

#include <SoftwareSerial.h>
SoftwareSerial SIM900(7, 8);

char incoming_char=0;
String inputString = "";         // a string to hold incoming data
boolean stringComplete = false;  // whether the string is complete

void setup()
{
  Serial.begin(19200); // for serial monitor
  inputString.reserve(200);
  SIM900.begin(19200); // for GSM shield
}

void loop()
{
  serialEvent(); //call the function
  // print the string when a newline arrives:
  if (stringComplete) {
    inputString = inputString + "\r";
    SIM900.print(inputString);
    // clear the string:
    inputString = "";
    stringComplete = false;
  }

  // Now we simply display any text that the GSM shield sends out on the serial monitor
  if(SIM900.available() >0)
  {
    incoming_char=SIM900.read(); //Get the character from the cellular serial port.
    Serial.print(incoming_char); //Print the incoming character to the terminal.
  }
}

void serialEvent() {
  while (Serial.available()) {
    // get the new byte:
    char inChar = (char)Serial.read();
    // add it to the inputString:
    inputString += inChar;
    // if the incoming character is a newline, set a flag
    // so the main loop can do something about it:
    if (inChar == '\n') {
      stringComplete = true;
    }
  }
}