Problem with serial communication between Arduino UNO and GSM Shield...

Hy everybody… Recently I purchased a GSM Shield (with SIM900 module) but I’ve some problems with serial communication between Arduino and the shield… I uploaded this sketch on Arduino:

#include <SoftwareSerial.h>

SoftwareSerial GPRS(7, 8); //Seriale simulata per comunicare col modem

unsigned char buffer[64]; // buffer per i dati scambiati sulla seriale
int count=0;              // contatore dati nel buffer

void setup()
  GPRS.begin(19200);   // Baudrate di default del modem SIM900
  Serial.begin(19200); // Baudrate della seriale di Arduino 

void loop()
  if (GPRS.available())  // Se ci sono dati sulla seriale simulata (il modem ci invia dati)
    while(GPRS.available()) // Finché ci sono dati disponibili
      buffer[count++]; // Inserisce i byte nel buffer
      if(count == 64)break;
    Serial.write(buffer,count);  // Scrive i dati ricevuti sulla seriale di Arduino
    clearBufferArray();  // Svuota il buffer
    count = 0;           
  if (Serial.available()) // Se ci sono dati sulla seriale di Arduino
    GPRS.write(; // Inviamo i dati al modem
void clearBufferArray() // Funzione di supporto per svuotare il buffer
  for (int i=0; i<count;i++)

then I tried to use AT Command Tester website… I pressed Find port button, then I selected the Arduino COM; I setted the Baud rate on 19200 and then I pressed Connect button… Here is the problem because i received this error:

Found ports :COM1, COM3

No port is connected.

Sending AT query..

Invalid or no response from the device. Please check the modem port and the baud rate.Disconnecting port COM3.

I tried to use all available Baud Rate but I always received the same error…

Can anyone help me to solve this problem?

Thank all…

P.S: Sorry form my english, but I am italian… :blush:

Which Arduino do you have?
Which GSM shield do you have?
Which pins are you using to communicate between the Arduino and shield?