public partial class rs232 : Form
    {
        string InputData = String.Empty;
        
        // This delegate enables asynchronous calls for setting
        // the text property on a TextBox control:
        delegate void SetTextCallback(string text);

        public rs232()
        {
            InitializeComponent();

            // Nice methods to browse all available ports:
            string[] ports = SerialPort.GetPortNames();

            // Add all port names to the combo box:
            foreach (string port in ports)
            {
                cmbComSelect.Items.Add(port);
            }
        }

        private void cmbComSelect_SelectionChangeCommitted(object sender, EventArgs e)
        {
            if (port.IsOpen) port.Close();
            port.PortName = cmbComSelect.SelectedItem.ToString();
            //stsStatus.Text = port.PortName + ": 9600,8N1";

            // try to open the selected port:
            try
            {
                port.Open();
            }
            // give a message, if the port is not available:
            catch
            {
                MessageBox.Show("Serial port " + port.PortName + " cannot be opened!", "RS232 tester", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                cmbComSelect.SelectedText = "";
               // stsStatus.Text = "Select serial port!";
            }
        }

        private void btnSend_Click(object sender, EventArgs e)
        {
            if (port.IsOpen) port.WriteLine(txtOut.Text);
            else MessageBox.Show("Serial port is closed!", "RS232 tester", MessageBoxButtons.OK, MessageBoxIcon.Error);
            txtOut.Clear();
        }

        private void btnClear_Click(object sender, EventArgs e)
        {
            txtIn.Clear();
        }

        private void port_DataReceived_1(object sender, SerialDataReceivedEventArgs e)
        {
            InputData = port.ReadExisting();
            if (InputData != String.Empty)
            {
//             txtIn.Text = InputData;   // because of different threads this does not work properly !!
                SetText(InputData);
            }
        }

        
        private void SetText(string text)
        {
            // InvokeRequired required compares the thread ID of the
            // calling thread to the thread ID of the creating thread.
            // If these threads are different, it returns true.
            if (this.txtIn.InvokeRequired)
            {
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke(d, new object[] { text });
            }
            else this.txtIn.Text += text;
        }

        private void ok_button(object sender, EventArgs e)
        {
          

        }