I'm trying to test AT commands (I am working with an Arduino Uno and a GSM/GPRS SIM900 shield; my system is ubuntu 14.10) . Yesterday when I was testing , all ran well.. For example after typing AT+SAPBR=3,1,"APN","ooredoo" in the serial monitor it showed me OK.. but But now it shows me strange characters.

This is my code

    #include <SoftwareSerial.h>
    SoftwareSerial GPRS(2,3);

void setup() {
  // put your setup code here, to run once:


void loop() {

  // put your main code here, to run repeatedly:



And this it is the result of AT+SAPBR=3,1,"APN","ooredoo" typed in the serial monitor

enter image description here

Thank you for helping me

  • Did you select the right baud rate? I get these strange characters when I have set it wrong.
    – Juergen
    Commented Jan 23, 2017 at 16:55
  • i didn't change anything...it worked like this yesterday,but now i have this strange characters...what can be the problem?
    – EL General
    Commented Jan 23, 2017 at 21:16
  • You haven't changed any settings, but have you knocked a wire. It could be a loose connection or a bad ground connection. Commented Jan 24, 2017 at 8:37
  • bad connection? i don't know..but the shield work normaly..the red led is on, the station led is also on and the net led blind every 3 seconds
    – EL General
    Commented Jan 24, 2017 at 12:27

1 Answer 1


for those who have this issue, i resolve this by changing the speed baud of my GPRS Shield :


and if you want to test AT command directly on the serial monitor you must use 2400 anywhere


and don't forget to change also the speed on the serial monitor to 2400 I hope this will help someone

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