Gallentro:
hooked up the serial connection and started my serial monitor and turned on. As expected the chip sent out some data upon turning on, but it looked like this: Ý Ý Ý Ý Ý Ý .Trying to communicate with it through AT commands did nothing at all, no reply. I see from the blinking that it has registered with the network, but it's not reachable through the serial connection.
I had this problem. Curiously if I turned the shield off my serial monitor showed NORMAL POWER DOWN which nobody mentioned, but that probably doesn't matter. I just found the fix. (and it doesn't require a Teensy) I have a Linksprite shield that I've been using pins 7 and 8 for software serial on, so if you're using pins 2 and 3 make sure to change it back from my code. Here's how I got it to stop outputting Ý Ý Ý Ý Ý Ý:
//Serial Relay - Arduino will patch a
//serial link between the computer and the GPRS Shield
//at 19200 bps 8-N-1
//Computer is connected to Hardware UART
//GPRS Shield is connected to the Software UART
#include <SoftwareSerial.h>
SoftwareSerial mySerial(7, 8);
void setup()
{
mySerial.begin(19200); // the GPRS baud rate
Serial.begin(19200); // the GPRS baud rate
mySerial.println("AT+IPR=19200"); // Tell the SIM900 not to autobaud
}
void loop()
{
if (mySerial.available())
Serial.write(mySerial.read());
if (Serial.available())
mySerial.write(Serial.read());
}
I can now open the serial monitor and issue AT commands and the SIM900 module responds the way I would expect. (Saying OK to certain commands, taking input for commands like sending an SMS, etc.) A new issue I found is that I don't know how to send Control + Z from the Arduino Serial Monitor. To work around this, I opened Terminal on my Mac and established the same type of connection there:
~ $ screen /dev/tty.usbmodem1421 19200
From that I can send ASCII code 26 (Control + Z) and I successfully sent an SMS by typing in each AT command needed to do so. I've been following the excellent tutorial at http://tronixstuff.com/2013/09/18/tutorial-arduino-sim900-gsm-modules/ but now I can do much more because I can just type things into Terminal. Hope this helps somebody!