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I needed a way to send SMS messages from a Netduino board, so I first tried the SM5100B shield. It worked somehow but it was not very stable and I did not like the hardware design. It also lacked some features. So I decided to build my own. Here is what I did (if you already used Arduino/Netduino the nomenclature below will be familiar, if not read a little bit about Arduino or Netduino, its a really nice microcontroller platform):
I also added a battery holder for backing up the RTC in the module, so you do not lose the time stored in the module. Also very handy.
The SIM card holder is a push-lock and is on the bottom of the PCB facing to the side with the usb and power connector, so when a housing is used you can cut a slot into the housing and change the sim-card without opening it.
The USB port can be used to configure the GSM board, but this is optional. All settings can be made through the RS232 interface.