c# - SMS by AT commands with GSM mobile is giving error

Question

I am sending SMS by AT commands with GSM mobile phone. I wanna send bulk of message like thousands. I read that by GSM mobile we can send 6-8 sms per minute. But when I send messages then someone are going and someone not. I am getting information from excel file means destination number and message text. Can you tell me why some sms are going and some not. My code is

        SmsFields smsObj = null;
        List<SmsFields> smsColl = null;
        SerialPort serialport = null;
        StringBuilder strbuild = new StringBuilder();
        try
        {
            //Validate the form 
            if (!Validation()) return;

            serialport = new SerialPort();

            ////Sets the properties of serial port object
            serialport.PortName = cboPort.SelectedItem.ToString();
            serialport.BaudRate = 9600;
            serialport.Parity = Parity.None;
            serialport.DataBits = 8;
            serialport.StopBits = StopBits.One;
            serialport.Handshake = Handshake.RequestToSend;
            serialport.DtrEnable = true;
            serialport.RtsEnable = true;

            //Open the port to send sms
            serialport.Open();

            //Check if port is opened or not
            if (!serialport.IsOpen)
            {
                MessageBox.Show("Serial port is not opened. Please try with other port");
                return;
            }

            //Create smsFields class's object and fill the data in the generic collection
            smsObj = SmsFields.Instance;
            smsColl = smsObj.FillData(txtFilePath.Text);

            if (smsColl == null)
            {
                MessageBox.Show("No data found in the excel table");
                return;
            }
            //Gets the single record from SmsFields class and sends the message
            foreach (SmsFields sms in smsColl)
            {

                //checks phone status
                serialport.WriteLine("AT" + Environment.NewLine);
                //Configures message as SMS (0 for PDU format) and (1 for text format)
                serialport.WriteLine("AT+CMGF=1" + Environment.NewLine);

                //Sets message center number
                serialport.WriteLine("AT+CSCA="" + txtServiceNo.Text + """ + Environment.NewLine);

                //Sets destination number
                serialport.WriteLine("AT+CMGS="" + sms.DestinationNo + """ + Environment.NewLine);

                //Specifies message and sends Ctrl+z
                serialport.WriteLine(sms.Message + (char)26);

                //Displays buffer containing output messages
                System.Threading.Thread.Sleep(4000);
   }

Answer 1

I think your problem is that you are not waiting for the final result code (i.e. OK, ERROR and a few others) before sending the next command. The problem with that is that the new command will trigger an abort of the ongoing command if it is not finished. To quote V.250:

5.6.1 Aborting commands

...

Aborting of commands is accomplished by the transmission from the DTE to the DCE of any character.

So ALWAYS when sending AT commands, you MUST wait for the final result code before sending the next command.

Might I suggest refactoring serialport.WriteLine("ATxxx" + Environment.NewLine) into a sendCommand(serialport, "ATxxx") function? And then you can add waiting for the final result code at the end of that function.