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I am using GSM 900A modem which requires 5V Supply. I am connecting it to Arduino UNO. I am giving supply to modem by arduino 5V and GND pin. I am connecting RXD pin to TX(pin 1) of arduino TXD pin to RX(pin 0) of arduino and GND to GND of arduino i.e. pin 14. I am running the basic example codes but GSM is not responding. I have also tried other softwares like Putty but I am unable to write any AT command Please help me. When I tested the modem using this code:

/*
 This example tests to see if the modem of the
 GSM shield is working correctly. You do not need
 a SIM card for this example.
 Circuit:
 * GSM shield attached
 Created 12 Jun 2012
 by David del Peral
 modified 21 Nov 2012
 by Tom Igoe
 http://www.arduino.cc/en/Tutorial/GSMToolsTestModem
 This sample code is part of the public domain
 */
// libraries
#include <GSM.h>
// modem verification object
GSMModem modem;
// IMEI variable
String IMEI = "";
void setup() {
 // initialize serial communications and wait for port to open:
 Serial.begin(9600);
 while (!Serial) {
 ; // wait for serial port to connect. Needed for Leonardo only
 }
 // start modem test (reset and check response)
 Serial.print("Starting modem test...");
 if (modem.begin()) {
 Serial.println("modem.begin() succeeded");
 } else {
 Serial.println("ERROR, no modem answer.");
 }
}
void loop() {
 // get modem IMEI
 Serial.print("Checking IMEI...");
 IMEI = modem.getIMEI();
 // check IMEI response
 if (IMEI != NULL) {
 // show IMEI in serial monitor
 Serial.println("Modem's IMEI: " + IMEI);
 // reset modem to check booting:
 Serial.print("Resetting modem...");
 modem.begin();
 // get and check IMEI one more time
 if (modem.getIMEI() != NULL) {
 Serial.println("Modem is functoning properly");
 } else {
 Serial.println("Error: getIMEI() failed after modem.begin()");
 }
 } else {
 Serial.println("Error: Could not get IMEI");
 }
 // do nothing:
 while (true);
}

I am getting this output on Serial Monitor for 9600 baud rate:

Starting modem test...ERROR, no modem answer. Checking IMEI...Modem's IMEI: 0 Resetting modem...Modem is functioning properly