RhydoLABZ’s SIM900A GSM/GPRS MODEM -RS232 – RMC is same as the Part No: GSM-1964  except the RMC Connector is populated instead of DB9 Connector.The Modem  is built with Dual Band GSM/GPRS engine- SIM900, works on frequencies 900/ 1800 MHz. The Modem is coming with RS232 interface,which allows you connect PC as well as microcontroller with RS232 Chip(MAX232). The baud rate is configurable from 9600-115200 through AT command. The GSM/GPRS Modem is having internal TCP/IP stack to enable you to connect with internet via GPRS. It is suitable for SMS, Voice as well as DATA transfer application in M2M interface.

The Modem is manufactured with Automatic Pick and place machine with high quality standard.The onboard Low dropout 3A Power supply allows you to connect wide range unregulated power supply. Using this modem,you can make audio calls, SMS, Read SMS, attend the incoming calls and internet etc through simple AT commands.

Controller based embedded systems there might not be a DB9 connector for the RS232 standard,but the output still comes at RS232 logic level so in such cases it is better to use modem available with RMC connector with other end having simply loose wires which we can  either solder to board or else we can easily think of any other good idea to bring connectivity to our system by simply having a glance at it!

 The SIM900A is a complete Dual-band GSM/GPRS solution in a SMT module which can be embedded in the customer applications 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.

Note:This Modem is meant for Asian countries only


  • High Quality Product (Not hobby grade)
  • RS232 interface @ RMC Connector for direct communication with computer or MCU kit
  • Configurable baud rate
  • SMA connector with GSM Antenna.
  • SIM Card holder.
  • Built in Network Status LED
  • Inbuilt Powerful TCP/IP protocol stack for internet data transfer over GPRS.
  • Audio interface Connector
  • Normal operation temperature: -20 °C to +55 °C
  • Input Voltage: 4.5V-12V DC

Pin Diagram:

Pin Details:


  • Make sure your SIM card is unlocked.
  • Default factory  baudrate is 9600.
  • Input Voltage: 4.5V-12V DC.

Getting Started:

  • Insert an unlocked SIM card to SIM card Holder. 6 Pin Holder for SIM Cards is provided on the Modem.

  • Close SIM card Holder.Both 1.8 volts and 3.0 volts SIM Cards are supported by SIM900A – the SIM card voltage type is automatically detected.

  • Assemble GSM Antenna to the modem
  • Connect serial cable to the modem
  • Give power supply in between  4.5V to 12V through the power jack provided
  • Default factory Baudrate is 9600
  • When the modem is successfully powered-up, the Red LED on the modem (PWR) will be ON, the STS LED (GREEN) will light after 1-2 seconds and the NET LED(BLUE) will blink every second. After the Modem registers in the network (takes between 10-60 seconds), this LED will blink in step of 3 seconds.

LED status description:
The Network LED indicates the various status of GSM module eg. Power on, Network registration & GPRS connectivity. When the modem is powered up, this NETWORK LED will blink every second. After the Modem registers in the network (takes between 10-60 seconds), this LED will blink in step of 3 seconds. At this stage you can start using Modem for your application,showing that modem is registered with the network.

How to Test:

1. Check your GSM/GPRS Modem Using Hyper terminal in PC:

  •  You can’t connect this Modem directly with your PC,you can make connection by soldering DB9 female connector at the end of the RMC connector wire as shown below.

  • Connection between DB9 and Modem is as given below.
    • We have to connect RTS pin of DB9 to RTS pin of the modem
    • CTS pin of DB9 to CTS pin of the modem
    • RD pin of DB9 to TXD pin of the modem
    • TD pin of DB9 to RXD pin of the modem 
    • GND pin of DB9 to GND pin of the modem.
  • Connect 330 ohm resistance in between CTS and RTS pin of DB9 connector.
  • RS232 interface @ RMC Connector for direct communication  MCU kit.
  • One sample test is shown using rhydoLABZ GSM/GPRS RS232 MODEM-SIM900A(RMC). For making connection with PC here we used serial cable.
  • Once you are done with all these procedures, choose the appropriate COM port that got assigned to the Communication Port in your system by looking into the device manager like as shown below.

  • In case we are having window XP OS and not having window terminal in it, then we need to create a new window terminal for checking our Modem using PC.

Creating a Hyperterminal

  • If you are using Windows XP, you can create a new Hyperterminal as shown below:

  • There could be some windows as shown below ,You may set and skip as per the highlightings:

  • Now you can give an appropriate  name to the connection like 9600 and click OK.

  • Also Select the appropriate COM port when asked for, in our case most probably it would be COM1.

  • Ensure that you have kept the settings like Baud rate  which should be 9600, parity none ,data bits 8 and stop bit 1 and flow control can also be kept as none.

  • You can simply use Hyper Terminal. After you click apply and OK  you will be shown the window below,By default it would be disconnected, Simply Click on the Highlighted button to connect.

  •  Now let’s try to check our Modem with AT commands for that we need to send following AT commands to Modem as shown  below.

  • The above shown command :AT command (followed by enter) after “OK” response, this signifies that our Modem is working properly.ATE0 command (followed by enter) is being sent to stop the echo. AT+CREG? (followed by enter) is being used to check whether the SIM got registered or not.
  • To get complete application note on AT commands click here.

Sending a text message from PC using AT Commands and Hyperterminal

  • Now next let’s send a message using rhydolabz GSM/GPRS RS232 MODEM-SIM900A(RMC). For sending message we need to first send related AT commands to initialize the modem to send a message.

  •  The AT+CMGF=1 command (followed by enter) sets the GSM Modem in SMS text mode.
  • AT+CMGS=<number>followed by enter gives<CR><write message><Ctrl+z>
    <CR> represents the carriage return character.
    <Ctrl+z>When you finish entering the SMS message body, you have to enter the <Ctrl+z> character to mark the end of the SMS message body.
    +CMGS: x with OK response ; where x = “current number of sent SMS messages”

Receiving a text message in Modem and displaying in Hyperterminal window

  • Now let’s receive a message using rhydolabz GSM/GPRS RS232 MODEM-SIM900A(RMC).For receiving message we need to first send related AT commands to initialize the modem.

  • AT+CNMI=2,1,0,0,0 command (followed by enter) set the modem to indicate received messages with +CMTI response indicates that new message has been received and shows location number of last received message  in above figure it is shown by number ’9′.
  • AT+CMGR=<index> :<index>integer type; value in the range of location numbers supported by the associated memory and gives +CMGR response which includes status i.e “REC UNREAD” Received unread messages,Sender’s number,date,time and received message.

Receiving a voice call from Modem using AT Commands

  • For receiving call we need headset,connector jack and modem.
  • Headset is connected in the marked pins of GSM Modem

  • Connections are as shown below
    • Connect MIC of Modem to MIC of Jack
    • Connect SPK of Modem to SPK of Jack
    • Connect GND of Modem to GND of Jack

  • For receiving a call  firstly we need to send some AT commands as shown below.

  • Loudspeaker Volume Level

    – the value of max can be read by issuing the Test command

    For example:

    -Command is used to select the volume of the internal loudspeaker audio
  • RING indicates incoming call.
  • For Attending call we are using ATA command (followed by enter) gives “OK” response when call is connected.
  • For Disconnecting call we are using ATH command  (followed by enter) gives “OK” response when call is disconnected.

Check your GSM/GPRS Modem Using Online AT Command Tester

  • AT Command Tester   is an online tool for testing AT commands and other module functionalities such as data call, voice call,HTTP, network registration, SMS, Phonebook, signal quality, SIM interface, device info etc.
  • Firstly you need to connect your GSM Modem with your system.
  • Here some example are shown using AT Command Tester
  • First select ‘Find Ports’ to search for available ports. Then select the modem port and press ‘Connect’. Once connected to the module, you can send AT commands from the ‘Command Mode’ tab

  • Sending AT commands using AT Command Tester

  • AT command (followed by enter) after “OK” response, this signifies that our Modem is working properly.ATE0 command (followed by enter) is being sent to stop the echo.
  • In script mode, you can send multiple AT commands at a same time as shown below. You can create your own script and save it on the local machine.Then load the script and execute.

  • In script mode we can save necessary AT commands and if required than we just need to load that saved script and by clicking on Run Script we will get output in column provided at right side of the tester.
    • For saving script click on save script (in right side column its shown where your script got saved) as shown below

    • For loading script click on load script (in right side column loaded script path is shown)  then click on  Run Script and output is as shown below in right column.

  • To perform basic diagnostics on the SIM900 module, choose the ‘Diagnostics’ tab.

  •  Select the ‘Voice Call’ tab to test outgoing and incoming voice calls.The ‘AT Command Tester’ will check whether the device is registered before dialing the call.

  • Select the ‘SMS’ to send SMS messages. You can also read and delete SMS messages stored on the SIM.

2.Testing with Arduino
  • For making connection between GSM Modem and Arduino you may use RS232 – TTL ( 5V) Converter.
  • While interfacing GSM Modem with Arduino you may need level shifter (MAX232 IC) because Arduino and GSM Modem works at different logic levels, connection diagram is shown below.
  •  Sample code below shows how we can send a message using Arduino interfacing with modem

3.Testing With PIC16f877

  • When interfacing   GSM Modem with PIC 16F877A need level shifter (max232 ic) because microcontroller and GSM Modem are different logic levels,connection diagram is shown below.

  • Sample code below shows how we can send a message using PIC 16F877A interfacing with modem


How to buy:

Click here to buy GSM/GPRS RS232 MODEM-SIM900.


Please share your ideas with us, visit our forum for discussion.

Frequently Asked Questions(FAQ):

Q. How much voltage is being supplied to the module ?
Ans. Input voltage given to the module is from 5V-12V.

Q. Why Antenna is needed in GSM modem ?
Ans. Antenna is needed to increase gain level and to establish a communication.

Q. Why sometimes we are not able to hear properly in Headset connected to the modem ?
Ans. This problem arises because some times we forget to set loudspeaker volume.
loudspeaker volume is set by following AT command
0..max – the value of max can be read by issuing the Test command
For example:
AT+CLVL=100 -Command is used to select the volume of the internal loudspeaker audio.

Q. What the three LEDs indicate in the GSM modem ?
Ans. PWR LED(Power LED)  – Red LED will lit immediately
STS LED(Status LED)  – Green LED will lit after 1-2 seconds
NET LED (Network LED) -Blue LED will blink fast for few seconds(Searching  For Network) and becomes slow blinking once  the Modem registers with the Network.

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