Interfacing SIM900A using PUTTY.exe

This GSM modem can accept any GSM network operator SIM card and act just like a mobile phone with its own unique phone number. Advantage of using this modem will be that you can use its RS232 port to communicate and develop embedded applications. Applications like SMS control, data transfer, remote control and logging can be developed easily.



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The modem can either be connected directly to the PC serial port or to any microcontroller through MAX232. It can be used to send and receive SMS or to make/receive voice calls. It can also be used in GPRS mode to connect to the internet and do many applications for data logging and control. In GPRS mode you can also connect to any remote FTP server and upload files for data logging.



The product can be purchased from this link.

Software description

To communicate with the network we use putty software. It is an executable file which supports serial data transmission and reception similar to HyperTerminal in Windows XP.

How to connect to computer ?

Firstly insert the SIM card in the sim holder. Make sure to use a normal SIM card (Micro or nano SIM cards are not supported)

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Connect the GSM modem to the usb2serial hardware via DB9 connector. Then using a USB A to B cable connect to the computer.

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  • Connect a 12V/9V 1A adapter to the power jack provided on the GSM modem.  The LM317  voltage regulatorregulates the voltage  to 3.6V.
  • Switch ON the modem.
  • Please note when you connect the power, a RED LED will glow.  It shows that the modem has got the power required for its operation.
  •  When you connect the power , a BLUE LED will glow for approximately 1sec.  After that it will blink  with a delay of approximately half sec. That means the SIM card  is searching for  network.  If the SIM card has connected to a network, then the BLUE LED will blink with a delay of approximately 4sec. Depending on the network strength, the time to connect to a network may vary. If there is no SIM card in the modem, the BLUE LED blinks with a delay approximately half sec.

The steps required for connecting the modem to the PC is shown below:

Right click My Computer → select the Device manager → Ports(COM & LPT)computer_management
Double click ‘putty.exe’→select serial bullet→change the com port number to the number shown earlier in the device manager →click open

Then a new window is opened and we give the commands in the window to communicate with the GSM modem and the other network through GSM modem. The commands are called ‘AT commands’ or ‘Attention commands’. To check the connection type ‘AT’ in the putty command window and press enter. If there is a connection it displays ‘OK’, and ‘error’ for no connection.

Please note if SIM is not inserted then it will show “+CPIN: NOT INSERTED”.

To make a call type ATD<phone number>;
e.g.: ATD9020XXXXXX;

If a call is coming to the GSM modem, a series of “RING” will appear on the window. If you want to connect to the call ,then type “ATA” & press enter.
To send a message, first you may have to change the message format from PDU to TEXT by typing AT+CMGF=1;&W . To send the message type the following
AT+CMGS=’’9020XXXXXX” and press ENTER
After this, you can see the message prompt (>). Then type the message and press Ctrl+z to send the message.

Or press Esc to abort.
You can see that after the message has been sent, the modem gives a response”+CMGS: xx″. From this you can confirm that message has been sent successfully.

eg: “+CMGS: 23”

The number 23 means 23 messages has been sent from the modem.


Which power adapter should I use?

You can use 9V or higher with ampere rating 1A or more. If you use adapter with ampere rating less than 1A there is a chance of modem getting reset when making a call or sending a message, because even though rated at 1A, it may supply only around 800mA.

My modem is showing ‘yyyy’ recurringly?

This is because the modem is getting reset, most probably a power issue. Check the connection for any loose contact. Also check if the power  supply is working properly. Consider changing the power adapter with more ampere rating.

No ports are shown in the software after scanning ports. Why?

The USB to UART driver has not been installed correctly or the hardware was not correctly connected to the computer. This can be confirmed by checking for the USB to UART port information in the device manager.

My modem is not connecting to a network?

One reason is the network strength in your area is poor. Move the modem to an area with more network strength.

Another reason is when you put a SIM of different service provider in the modem for the first time. In that case take the SIM back from the modem and reinsert it into your phone and connect to the network. Power OFF the GSM modem. After connecting to the network, take the SIM from the phone and then reinsert it into the modem. Power ON the GSM modem. This should work.

I cannot see anything I type in the modem ?

One reason is that the baud rate is different. The computer and the modem should be set at the same baud rate for proper communication. Try opening modem test utility or putty with different baud rates and try any AT command.For example if the modem is set at 115200 baud rate and computer(modem test utility) is set at 9600 baud rate then it will not communicate. Try opening modem test utility with different baud rates . So when the modem test utility is set at 115200 it will communicate.

Another reason is that ECHO is off. You can easily know the echo is off because the modem responds to your AT commands but the only problem is that you cannot see what you are typing. To turn on echo type ATE1&W<press ENTER>.

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