What is the best design for polling a modem for incoming data?

Question

I have a GSM modem connected to my computer, i want to receive text messages sent to it using a python program i have written, am just wondering what is the best technique to po

Answer 1

I find I can't remember much of the AT command set related to SMS. Andre Miller's answer seems to ring a few bells. Anyway you should read the documentation very carefully, I'm sure there were a few gotchas.

My recommentation for polling is at least every 5 seconds - this is just for robustness and responsiveness in the face of disconnection.

I used a state machine to navigate between initialisation, reading and deleting messages.

Answer 2

I have written something similar before. There is a way using AT commands to tell the modem to signal you each time an SMS is received.

For reference, I was using a Maestro 100 GSM Modem in an embedded application.

First you have to initialize the modem properly. I was using text mode for the SMS, but you might be using something different. Pick from these what you want. AT+CNMI is the most important.

AT&F0 # Restore factory defaults
ATE0  # Disable command echo
AT+CMGF=1 # Set message format to text mode
AT+CNMI=1,1,0,1,0 # Set new message indicator
AT+CPMS="SM","SM","SM" # Set preferred message storage to SIM

You would then wait for a message notification, that will look like this. (Don't match on the index number, that might differ between notifications)

+CMTI: "SM",0 # Message notification with index

When you get that notification, retrieve the unread SMS's:

AT+CMGL="REC UNREAD"  # Retrieve unread messages

I would recommend you also add a poll, maybe every 5 minutes or so, just in case you miss a notification. With serial comms you can never be sure!