User Manual for GSM

User Manual for GSM

Tags: Sim900a gsm modem user manual,sim900a schematics,at commands data sheet, sim900a at commands,,
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SIM900A overview

SIM Com presents an ultra compact and reliable wireless module. The SIM900A is a complete Dual-band GSM/GPRS module in a SMT type which is designed especially for Chinese market, allowing you to benefit from small dimensions and cost-effective solutions.

Featuring an industry-standard interface, the SIM900A delivers GSM/GPRS 900/1800MHz performance for voice, SMS, Data, and Fax in a small form factor and with low power consumption. With a tiny configuration of 24mm x 24mm x 3 mm, SIM900A can fit almost all the space requirements in your applications, especially for slim and compact demand of design.


General Block Diagram

Power Supply

The external power can be AC or DC, with a voltage between (9V/12V, 1A output) at 230V AC input. The GSM Modem board produces +4.1V using an LM7805 voltage regulator, which provides supply to the GSM Module.

Light Emitting Diodes

Light Emitting Diodes (LEDs) are the most commonly used components, usually for displaying pin’s states.


Point LEDs

GSM Board

LED Selection

LED Indicators






Network signal

RS-232 Communication (UART)

  • RS-3202 communication enables point-to-point data transfer. It is commonly used in data acquisition applications, for the transfer of data between the microcontroller and a PC.
  • The voltage levels of a Modules and PC are not directly compatible with those of RS-232, a level transition buffer such as MAX3232 be used.


UART DB-9 Connector

GSM Module SIM900A

Serial Port Section



Pin-2 (TXD)



Pin-3 (RXD)


Getting Started

  • Insert SIM card: Press to remove the tray from the SIM cardholder. After properly fixing the SIM card in the tray, lock the tray in the slot provided.
  • Connect Antenna: Screw the RF antenna on the RF cable output provided.
  • Connect RS232 Cable to the Host device: (Cable provided for RS232 communication)
    Note: Default baud rate is 9600 with 8-N-1, no hardware handshaking.
  • Connect the power Supply (9V-AC/DC) to the power jack. In case of DC polarity should be Center +ve and outer –ve on DC jack.
  • Network LED indicating various status of GSM module eg. Power on, network registration & GPRS connectivity.
  • After the Modem registers the network, led will blink in step of 3 seconds. At this stage you can start using Modem for your application.
  • AT commands set section is covered in following document

Trouble Shooting

  • Note: when GSM modem powers on the auto baud is enabled, user must first to send the “A” to synchronize baud rate. It is recommended to wait 2 to 3 seconds before sending “AT” character. After receiving the “OK” response, PC and modem are correctly synchronized.
  • Serial cable (DB9 Male to Female) must be straight connection (2-2 & 3-3and 5-5)
  • Ensure the the sim card holder/ connector has the SIM card.
  • It is not advised to connect or disconnect any external devices which are not recommended by the product reference manual.
  • It is risky to connect power supply not preferred by the reference user manual
  • In case of any malfunction in the product, please let us know.



1. To check the modem:AT{Press Enter}



2. To change sms sending mode:AT+CMGF=1{Press Enter}



3. To Read SMS in Text mode:

AT+CMGR= no.

Number (no.) is the message index number stored in the sim card. For new SMS, URC will be received on the screen as +CMTI: SM ‘no’. Use this number in the AT+CMGR number to read the message.                                        


Example:             AT+CMGR=1{Press Enter}                                              

+CMGR: "REC READ","+917708283043","","10/05/21, 20:52:20+22"


4. To Send SMS in text mode :

AT+CMGS=”mobile number” press enter

Once The AT commands is given’ >’ prompt will be displayed on the screen.

Type the message to send via SMS. After this, press ctrl+Z to send the SMS.

If the SMS sending is successful, “ok” will be displayed along with the message number.


Example:             AT+CMGS=”9884467058”{Press Enter}

                                < Hello GSM Modem CTRL+Z




5. To make a voice call:ATD9884467058; {Press Enter}


6. To Redial last no:ATDL{Press Enter}


7. To Hang up or Disconnect a call: ATH{Press Enter}


8. For receive incoming call:ATA{Press Enter}


9. To set a particular baud rate:AT+IPR=?{Press Enter} {To view the baud rate values}

AT+IPR=0            {To set the modem to auto bauding mode}


10. Operator selection:AT+COPS? {Press Enter}




+COPS: 0, 0,"AirTel"


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