Why Doesn't My C# Code Send SMS via GSM Modem Using System.IO.Ports?

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

This article addresses a common problem: C# code using System.IO.Ports fails to send SMS messages via a GSM modem.

The core issue often lies in relying on Thread.Sleep() instead of properly handling modem responses. Robust SMS sending requires reading and interpreting the modem's feedback after each AT command.

Effective Solution

The V.250 standard (Chapter 5) provides best practices for AT command management. Crucially, it emphasizes using r for command termination, not Environment.NewLine.

For commands not requiring a response:

// Open serial port
serialport.Open();

// Send command
serialport.Write("AT+CMGF=1\r");

// Read and parse response
string line;
do {
    line = readLine(serialport); // Assumes a readLine function exists
} while (!is_final_result_code(line)); // Assumes an is_final_result_code function exists

The AT CMGS command, however, demands a specific response ("rn> ") before the SMS payload is sent. Waiting for this prompt is essential.

Reliable AT Command Handling

Replacing Thread.Sleep() with a mechanism that waits for the modem's final result code is vital for reliable AT command execution. This ensures accurate confirmation of command success or failure, resulting in a more dependable SMS system.

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