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I'm researching options for a project in which I need to send/receive SMS messages to/from multiple users of different service providers.
Hoping to reduce costs, I'm thinking of forwarding the messages to a different SIM card, depending of the user's service provider (since all of them have plans with free SMS for numbers of the same provider).
Arduino GSM Shield seemed like a good option, except the shield has only one SIM card slot.
I wonder: would it be possible to stack 3 shields (there are 3 mayor service providers in my country) on the same Arduino Board and use some reasonably straightforward programming to handle the task?
Thanks in advance.

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Hardware-wise this should be pretty easy to do. You need to connect pins 2 and 3 of each GSM shield to separate serial ports on the Arduino board. The best way to do this is to use an Arduino Mega, which has 4 hardware serial ports, this leaves you Serial for uploading sketches and communication to Serial Monitor. It's also possible that you could use a software serial library like AltSoftSerial but I wouldn't recommend it. You can carefully bend the pins 2 and 3 of two of the shields out before stacking them and then use jumper wires to connect those pins to the hardware serial pins of the Mega. The other pin of interest is pin 7, which is the modem reset pin. If you leave all the modems connected to pin 7 on the Arduino that means that there is no way to only reset a single modem. I'm not sure how much of an issue that would pose. So you might be better off to bend pin 7 on two of the shield out also and connect those to separate pins. You will also need to be sure to provide a power supply capable of supplying sufficient current. The product page says each shield may require up to 1000mA.

Software-wise it gets much more tricky. Unfortunately the stock Arduino GSM library was not written to be very flexible. Instead of allowing you to pass it a Serial object it's hardcoded to do software serial on pins 2 and 3 so you would need to do quite a bit of work on the library to make it work for your needs. I'd recommend looking around to see if a more suitable library exists.

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Stacking might be hard, since they communicate on pins 2&3. But if you work around that somehow, you could use Software Serial to access multiple GSM devices.

I think the GSM library has hardcoded the pins, and it might not share well with multiple, but you might be able to use SoftwareSerial or AltSoftSerial to do multiple connections, but that would be less straightforward.

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Yes, that is easy. You need multiplexer (MUX) to forward communication pins (2 & 3) to the right GSM shield. Google for it to find the one that suits your budget.

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