Unable to receive and print SMS on the serial monitor with Arduino Uno and SIM900A

Question

I'm trying to receive an SMS and show it on the serial monitor.

My code is as following:

void setup() 
{

  Serial.begin(2400);
  Serial.write("AT+CMGF=1\r");           //set GSM to text mode
  delay(1500);

  Serial.write("AT+CPMS=\"SM\"\r");         //Preferred SMS Message Storage
  delay(1000);

  Serial.write("AT+CMGL=\"ALL\"\r");       //List SMS Messages from Preferred Storage
  while(1)
  {
    if(Serial.available())
    {
      Serial.write(Serial.read());  
    }   
  }
}

void loop() 
{
}

The circuit diagram is as shown below.

The output that I am getting is as following:

AT+CMGR=1
AT+CMGF=1

OK
AT+CPMS="SM"

ERROR
AT+CMGR=1
AT+CMGF=1

OK
AT+CPMS="SM"

ERROR

ERROR
AT+CMGR=1
AT+CMGF=1

I am not able to use AT+CPMS or AT+CMGL commands.

Answer 1

As others have stated, you cannot use the hardware serial port for communication with the device and communication over the USB cable.

OPTION 1
Switch to a different microcontroller, such as the arduino mega with multiple hardware serial ports on board.

OPTION 2
Implement the Software Serial library for communication with the sim card device and use the hardware serial for communication with your computer.

Note: Software Serial implementations are more computationally intensive than the use of hardware serial. Do not expect the same performance from a software serial implementation and do not expect to be able to smoothly run multiple instances of it on a single 8 bit mcu.

Example Code (Untested):

  #include <SoftwareSerial.h>

  void setup() 
  {
  Serial.begin(9600);
  SoftwareSerial mySerial(10,11); //RX,TX pins
  mySerial.begin(2400);
  mySerial.write("AT+CMGF=1\r");           //set GSM to text mode
  delay(1500);

  mySerial.write("AT+CPMS=\"SM\"\r");         //Preferred SMS Message Storage
  delay(1000);

  mySerial.write("AT+CMGL=\"ALL\"\r");       //List SMS Messages from Preferred Storage
  while(1)
  {
    if(mySerial.available())
    {
      Serial.write(mySerial.read());  
    }   
  }
}


void loop(){}

Disconnect from pin 0 and pin 1 and connect to pins 10 and 11 respectively.

Answer 2

The Arduino UNO that you are using only supports one Serial connection (over the TX and RX pins). Since you are using these pins to talk to the device, you cannot also send data over Serial.

Read this article to see more about Serial and how it works on different boards.

Answer 3

The RX/TX pins are on Uno connected to the USB chip. If you connect a device to RX/TX, you should not use Serial Monitor. It sometimes works, but the device receives the data send to Serial Monitor too and can't process them.

Use SoftwareSerial library for connecting the serial device, if you want to have Serial free for printing to Serial Monitor over USB.