Know How : Using GSM Modem – Part 1/2

This document contains the study of GSM modem SIMCOM SIM300S, it briefly explains the process of communication with the modem using HyperTerminal. All the necessary AT Commands are discussed with syntax and example.


GSM Modem SIM 300 with TTL-700x700


  • Use AC – DC Power Adaptor with following ratings. DC Voltage : 12V /1A
  • Current Consumption in normal operation 250mA, can rise up to 1Amp while transmission.
  • Supports features like Voice, Data/Fax, SMS and GPRS.
  • Control via AT commands(GSM 07.07,07.05 and enhanced AT commands)


  • RS-232 through D-TYPE 9 pin connector
  • Serial port baud rate adjustable 1200 to115200 bps (9600 default)
  • BRK connector for MIC & SPK, SIM card holder
  • Power supply through DC socket
  • LED status of GSM / GPRS module



  • Plug-in the Power cable in the modem power input jack. Make sure the input voltage & current matches the power specifications of the module (12v Dc, 2amp).
  • Once the power is plugged in, the power LED would glow indicating availability of power.
  • Another LED (Red) besides the power LED would also start blinking indicating that GSM modem is working fine.
  • Now properly insert the SIM card in the SIM holder and plug out the power and reconnect it.
  • Wait for a minute and try calling the Phone number of the SIM inside the SIM holder.
  • If the number is not reachable or switched off, wait for some time and try calling again. Repeat and check connections if necessary.
  • When the ring goes the third LED (green) besides the DB9 connector will start blinking indicating an incoming call.
  • The testing is done and the modem is ready to be used.




  • For connecting our PC/Laptop and the GSM modem we require a serial cable or an Usb to serial cable respectively.
  • For PC there is no need of installing any driver the serial cable can be inserted on the serial port of the PC.
  • While using laptop insert the usb to serial converter to the laptop’s usb port. Let the system look for the driver software or download it manually and install it. Once installed connect the cable to Laptop.
  • Open the device manager in the laptop and click on Ports(COM&LPT)
  • Find if there is a USB serial Port. Note its com port (say COM4) for future reference.
  • Install the driver again properly if there is an ‘!’ sign besides the USB serial Port.




  • HyperTerminal comes pre installed in windows XP.  Open Start > All Programs > Accessories > Communications >HyperTerminal.
  • For windows Vista and windows 7 install a 30 day trial version of it from net.
  • Open the HyperTerminal application.










  • Name the connection say “GSM”, click OK
  • A new window would appear as follows


  • Select Connect Using as the COM port you noted from the device manager for your USB to Serial Cable. On PC serial port make it simply COM1. Leave other fields as default and click OK.
  • In the next window leave Values to default (make sure the baud rate is 9600) and click OK.













 Read part 2 of this Tutorial