How can I continuously write data to a text file?

Question

Every second I just want to write the DateTime(HH:mm:ss) and a 0 or 1 as a line in a text file.

0 appears if the received data does not contain UCAST and =g, if this is true, then I want to write the DateTime and 1. It's the DateTime where the message is received.

My problem is that I only get the DateTIme and 1, but never get DateTime and 0. The line is always replaced by the new DateTime and 1, so the old one doesn't exist any longer.

private void port_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
    {
        recievdata = SerialPort.ReadExisting();
        this.Invoke(new EventHandler(DisplayText));

        using (Stream stream2 = File.Open(path, FileMode.OpenOrCreate))
        using (StreamWriter sWriter1 = new StreamWriter(stream2))
        {
            if (recievdata.Contains("UCAST") && recievdata.Contains("=g"))
            {
                sWriter1.Write(DateTime.Now.ToString("HH:mm:ss"));
                sWriter1.Write(" ; ");
                sWriter1.WriteLine(1);
                if (SleepMovChar.InvokeRequired)
                {
                    SleepMovChar.Invoke(new MethodInvoker(delegate
                    { SleepMovChar.Series["Bevægelse"].Points.AddXY(xValue = DateTime.Now.ToString("HH:mm:ss"), 1); }));
                    SleepMovChar.ChartAreas[0].CursorX.IsUserSelectionEnabled = true;
                }
            }
            else
            {
                sWriter1.Write(DateTime.Now.ToString("HH:mm:ss"));
                sWriter1.Write(" ; ");
                sWriter1.WriteLine(0);
                timer1.Tick += new EventHandler(timer1_Tick);
            }
        }
    }