I've got the following module of SIM900a.

SIM90a Module

My connections are:

VCC -> External 5 Volt 1.5 Amp Supply
GND -> Common GND
TX -> Arduino RX (Pin 0)
RX -> Arduino TX (Pin 1)

I hear a ring when I call on the sim card that has inserted there. So it can register to a network successfully. I've uploaded a blank code in Arduino Uno to test AT Commands. But I'm not getting any response when I type AT on Serial Monitor. What am I doing wrong here?

  • Share some code with us! – SilvioCro Feb 4 '19 at 11:32
  • I've uploaded the blank code for testing AT commands. – opu 웃 Feb 4 '19 at 15:16
  • maybe the baud rate is incorrect – jsotola Feb 4 '19 at 17:22
  • I've tried all different baud rates. No luck :/ – opu 웃 Feb 4 '19 at 17:24

At first, make sure that the sim has connected to the network. For this, you can call to the sim number from another phone. If not ringing that means the sim wasn't able to connect with the network. For this check power supply, antenna etc. If ringing then there something wrong in connection or code. So cross check the connection and send AT to the module and wait for returned data. If no data returned, then there check connection again. If the returned data is OK, then it is ready to use. and if returned data is ERROR then Arduino have connected to sim module but sim network, power supply etc problem happening.

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  • I've already mentioned in the question that I can hear a ring back. – opu 웃 Feb 4 '19 at 17:02
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    I think you use hardware serial for communicating with pc for serial monitor and gsm. For this, it may conflict with each other. Try to use SoftwareSerial.h for gsm communication. – Takiuddin Ahmed Feb 4 '19 at 19:57

I was facing the same issue and solved this issue when I have used softwareSerial.h library and used to connect TXD of sim900A with pin 13 and RXD of sim900A withpin 12 of ardunio MEGA.

You can also use 10 and 11 pins of mega instade of 12 and 13 i.e.

#include <SoftwareSerial.h>      // Configure software serial por 
SoftwareSerial SIM900A(10, 11);  // (sim 900 connections RXD=>10 & TXD=>11)
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