Hello Adruino friends,

I would like to hook up a small project. In need this for resetting my firewall at home because it's doing strange stuff from time to time. I followed the instruction of the GSM shiel to hook it up and to see if I get this running. The goal is to send a SMS to my mobile phone. I should see if the test before hand are good. Unfortunately it's not doing well.


  • UNO replica board: mikrocontroller board with ATmega328P, ATmega16U2, compatible with Arduino UNO R3

  • GSM shield SIM 900

  • both boards have their own 5 V 10 W power supply

  • gnd connection between them

  • changed the jumper on the UNO for software serial

  • software serial connection on Pins 7 & 8 on UNO -> crossed connection to the GSM shield

  • use official Adruino IDE on Windows 10 and Debian

  • used this code to send SMS

  • changed TX und RX ports to 7 and 8 in GSM.ccp like:

//De-comment this two lines below if you have the
//second version og GSM GPRS Shield
//#define _GSM_TXPIN_ 2
//#define _GSM_RXPIN_ 3
#define _GSM_TXPIN_ 7
#define _GSM_RXPIN_ 8


  1. compiled and uploaded

  2. power both boards up

  3. hit power button on GSM shield
  4. look in serial monitor

Error: The GSM shield can't log into the net: The net LED indikcates this. On the serial monitor in the Adruino IDE I get this:

GSM Shield testing.
RIC: ⸮⸮⸮⸮
RIC: ⸮⸮⸮⸮
RIC: ⸮⸮⸮⸮
RIC: ⸮⸮⸮⸮
RIC: ⸮⸮⸮⸮

I could not find any final solution on my search. Do you have any ideas?

I appreciate any kind of ideas or crtitic.

Thanks Hirsch

  • Do you know whether the SIM900 works in your country or with your carrier? It doesn't work everywhere. – Majenko May 25 at 23:27
  • Good point. I didn't mention this. I will look. – Hirsch May 26 at 6:42
  • The Shield is a quadband modul. This means it works with 850/900/1800/1900 MHz. – Hirsch May 26 at 6:58
  • Here in germany all big carriers are sending in the GSM (2G) band at 900 MHz or 1800 MHz. My carrier is a reseller but it uses the O2 and the Vodaphone infrastrucure. So this should be fine. – Hirsch May 26 at 7:10
  • Looks like it should work fine. However the library you are using defaults to 9600 baud (github.com/MarcoMartines/GSM-GPRS-GPS-Shield/blob/GSMSHIELD/…) but the shield you link to defaults to 19200 baud (4) Select 19200 baud rate – the shield default setting is 19200...) so maybe modify the library to start at 19200? – Majenko May 26 at 10:01

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