Why Can't I Send SMS with C# and a GSM Modem Using System.IO.Ports?

Troubleshooting C# SMS Sending with GSM Modems via System.IO.Ports

This guide addresses common problems encountered when sending SMS messages using C# and a GSM modem through the System.IO.Ports library.

Avoid Sleep() for Response Handling

Never use Sleep() to wait for modem responses. Proper AT command interaction necessitates reading and parsing all responses, mirroring how HTTP clients must process server replies. Ignoring responses can lead to unpredictable behavior.

Consult the V.250 Standard

Chapter 5 of the V.250 standard provides essential guidance on AT command handling. Understanding this protocol is key to successful implementation.

Reliable AT Command Execution

For commands where detailed responses aren't critical, employ this robust method:

serialport.Open();
//...
serialport.Write("AT+CMGF=1\r");
string line;
do {
    line = readLine(serialport);
} while (!isFinalResultCode(line));
//...
serialport.Close();

Specific Handling for AT CMGS

The AT CMGS command requires unique handling. Before transmitting the message payload, wait for the modem's "rn> " prompt.

Further Points to Note:

• Terminate AT commands solely with r, not Environment.NewLine.
• Confirm correct modem detection and configuration.
• Utilize online resources and forums for targeted troubleshooting assistance.

Following these steps will significantly improve the reliability of SMS sending using C#, System.IO.Ports, and GSM modems.

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