I recently bought a SIM800a GSM module in India and I am unable to find the Rx Tx pins of the module to make calls using a Arduino duimilanove.

The main problem is also that I can't find the data sheet for Sim800a in the net so that I can use a multi meter to determine the Rx Tx pins.

Here are some screenshots of the module-

Top View showing 4 unlabeled pins compare with the labelled picture on the net Side View 1 Side View 2

There is no label showing Rx Tx here. There is I2C protocol(SCL,SDA) but I dont know how to code in Arduino for making a call using I2C.

I found a SIM800a picture on ebay with labels(not the data sheet) but when I tried using the pins in that manner it did not work. The SIM800a I have is connected to the network as the network led blinks after 3 sec and when I call it then the alarm rings. But it is unable to transmit calls or sms.

labelled sim800a

This is the code I am using. I remove the Rx Tx pins before uploading the code and immediately put them after uploading.

char phone_no[]="9953642386";
void setup()
Serial.begin(9600);  //Open Serial Connection at baudrate 9600
Serial.println("AT"); // Wake up GSM
Serial.print("ATD"); //Dial the phone number using ATD command
Serial.println(";"); // Semicolon is a must at the end
Serial.println("ATH"); // After a delay of 5 secs Hang the call
void loop()
// empty loop.If you enter the above code here,the call will be made FOR EVER repeatedly.
//Take Caution while coding under loop.At some condition you’ve to terminate the Call

Otherwise is it possible to use the SDA and SCL pins for this purpose of calling to a phone from the gsm module ? Any help would be appreciated.


This datasheet may help you. On page 14 you can see TXD and RXD are pins 9 and 10.

Orientation is hard to determine, but it appears that pin 1 is where the most red on the label is.

enter image description here You should now be able to trace where those pins go to. They look like they go to high numbered pins, around the 11/12 area.

  • Thanks found the pins :) didnt realize the data sheet of sim 800a and sim 800 will be the same
    – geek101
    Apr 22 '17 at 19:19

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