Hello!
Eltima team is starting the series of articles about using embedded devices with computers running OS X. Quite often we need to connect to devices remotely. Sometimes we need to send or get data, or to manage the device which is located far away and does not have Internet access.
To avoid inventing a new connection system, one can use already existing mobile coverage. We set our eyes on SIM900 - wireless GSM module, which is quite known on the market today.
The pros are primarily its affordable price (it costs something about USD 10.00) and simple usability. The radio module uses 5V input voltage and features low power consumption. It is stable in work and has operation temperature from -30°C to +85 °C. It is very compact in size (chip-size) and complies to CE, FCC, ROHS, PTCRB, GCF, AT&T and ICASA Certifications.
To get more info on SIM900, go to the manufacturer’s website:
http://wm.sim.com/defaulten.aspx
The module uses UART (universal asynchronous receiver/transmitter) for data transmission, therefore it can be easily integrated into any platform via standard serial interface. POSIX library on OS X, as well as on other UNIX systems, lets us use serial ports.
Once serial connection is established, interaction is ensured via AT commands, which we’ll study in detail in upcoming articles.
By sending AT commands to the module, one is able to:
- make outgoing calls and get incoming calls;
- get and send messages;
- create TCP connection or send HTTP requests to Internet via GPRS;
- send and get DTMF signals;
- use sound input/output for work with calls.
Note that there is various supplemented firmware which can expand the functional with additional features, like work with MMS, SMTP/POP3, PING, geolocation, etc..
So how can Sim900 be implemented? There are several variants:
- Alarm systems where events are sent via SMS messages or voice call. AT command should be sent in due time to make a call or send a text message to an end-user.
- Remote device management. Managing DTMF signals of the incoming call are received and transferred to the computer attached to the module.
Radio module usage is not limited to PC connection. One can connect it via UART to Atmel tiny microcontrollers, which, by the way, can be connected to the module via serial interface as well. In this case, in rather small form factor (2,5 х 2,5 cm).
It’s rather hard to find appropriate software to work with Sim900, since the functional should be specifically designed for integration into the definite infrastructure. Besides, advanced versions of Sim900 may differ. However, interaction via the serial interface can be carried out by terminal programs for modem management.
In our next articles, we’ll highlight the peculiarities of Sim900 integration in order to implement connection to remote devices. Stay tuned!
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