Test GSM modem

Projects General Guidance
1

I am trying to use my GSM SIM900A modem to send SMS through my arduino uno. I am not sure if the serial connections are working. I have connected the Rx and Tx of the modem to the Tx and Rx of the arduino respectively. How do i check if the modem is responding? I think i am supposed to send "AT" to the modem and get a response "OK". I am an amateur and i am not quite sure how to feed in the "AT" and where from to expect the response "OK".

I just need to know the modem is responding or not. How do i do it?

2

For testing purpose, you can connect the modem to PC and interface through hyperterminal...

for testing through arduino, you can write a simple program:

void setup()
{
pinMode(13, OUTPUT);
Serial.begin(9600);
}

void loop()
{
int i;
char a[2];
Serial. print("AT\r");
for(i = 0; i <2; i++)
{
a[i] = Serial.read();
delay(50);
}
if(a[0] == 'O' && a[1] == 'K')
{
digitalWrite(13, HIGH);
delay(1000);
digitalWrite(13, LOW);
delay(1000);
}
3

Thank you. This looks simple and is very clear. It should definitely work right? But its not working. The pin 13 is remaining low at all times. So is it certain the modem is not responding? Or should i try something else? I am getting a ringing tone with the SIM inserted and modem powered which means its reading the SIM. So the problem has got to be with the serial interface right?

bparth:
For testing purpose, you can connect the modem to PC and interface through hyperterminal...

for testing through arduino, you can write a simple program:

void setup()

{
pinMode(13, OUTPUT);
Serial.begin(9600);
}

void loop()
{
int i;
char a[2];
Serial. print("AT\r");
for(i = 0; i <2; i++)
{
a[i] = Serial.read();
delay(50);
}
if(a[0] == 'O' && a[1] == 'K')
{
digitalWrite(13, HIGH);
delay(1000);
digitalWrite(13, LOW);
delay(1000);
}

4

According to this video TX should be connected to TX, RX to RX. - Scotty

5

What product are you trying to connect to the Arduino? There are many (ok, a few!) that use the SIM900. A link would be useful.

6

scottyjr:
According to this video TX should be connected to TX, RX to RX. - Scotty

I have watched this video and have followed it basically. I think it depends on the modem whether i should connect Tx to Tx and Rx to Rx or vise versa. The modem i am using is this one

I think for this modem i should connect the Tx to Rx and the Rx to Tx.

I basically want to know how to transmit instructions to the modem and get the response back. For example if i send "AT" i am supposed to get "OK" as response as per the datasheet.

7

About half way down this http://www.seeedstudio.com/wiki/GPRS_Shield_V1.0 page is a script to load onto the Arduino. Connect the RX / TX pins to pins 7 & 8 of the Arduino and the Arduino to the computer, and then open the Serial Monitor. Don't forget that the modems do draw quite a bit of power when actually working, although it does remind you of that on the link you provided.

The script is a simple repeater. Whatever you type in the Serial Monitor will be sent to the SIM900, and anything received from the SIM900 will be relayed to the Serial Monitor. Check the baud rates before you begin.

8

Please be aware that the A-model only works in a few regions of the world, so unless you are there, you will be very disappointed!

http://www.blog.zapro.dk/?p=368

// Per.