Why Aren't My C# SMS Messages Sending Through My GSM Modem Using System.IO.Ports?

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

Many C# developers using System.IO.Ports to send SMS messages via GSM modems encounter a perplexing problem: messages fail to send, despite previous success. This article pinpoints the root cause and offers a robust solution.

The Problem: Over-reliance on Sleep Functions

The core issue stems from relying on sleep functions to wait for modem responses. This is unreliable. Instead of assuming success based on time, you must actively wait for and interpret the modem's feedback. This means reading and analyzing all modem responses.

Why Ignoring Modem Responses is a Critical Error

Neglecting modem responses after sending AT commands is analogous to ignoring server responses in HTTP requests – it compromises reliability and consistency. The V.250 standard (Chapter 5) provides detailed guidance on AT command handling, highlighting the critical role of termination characters, specifically r (carriage return).

The Reliable Solution: Active Response Handling

To guarantee reliable SMS transmission, implement active response handling:

<code class="language-csharp">serialport.Open();
...
// Send AT+CMGF=1 command
serialport.Write("AT+CMGF=1\r");
string line;
do {
    line = readLine(serialport); //Custom function to read a line from serial port.
} while (!is_final_result_code(line)); //Custom function to check for final result code.
// AT+CMGF=1 command processing complete (success or failure)
...
serialport.Close();</code>

Handling AT CMGS: A Specialized Approach

The AT CMGS command requires a unique approach. Before sending the SMS payload, you must wait for the rn> prompt from the modem. This ensures the modem is ready to receive the message.

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