C#

SerialPort serialPort1;
string RxString;

public frmMain()
{
    InitializeComponent();
}

private void btnStart_Click(object sender, EventArgs e)
{
    serialPort1 = new SerialPort() { PortName = txtPortName.Text, BaudRate = Convert.ToInt32(txtBaudRate.Text), DataBits = 8, Parity = Parity.None, StopBits = StopBits.One, Handshake = Handshake.None };

    if (!serialPort1.IsOpen)
    {
        serialPort1.Open();

        serialPort1.Write("SI\r\n");

        btnStart.Enabled = false;
        btnStop.Enabled = true;
        textBox1.ReadOnly = false;
    }
}

private void DisplayText(object sender, EventArgs e)
{
    textBox1.Text = "OK";
    textBox1.AppendText(RxString);
}

private void serialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    RxString = serialPort1.ReadExisting();
    this.Invoke(new EventHandler(DisplayText));
}

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    // If the port is closed, don't try to send a character.
    if (!serialPort1.IsOpen) return;

    // If the port is Open, declare a char[] array with one element.

    char[] buff = new char[1];

    // Load element 0 with the key character.
    buff[0] = e.KeyChar;

    // Send the one character buffer.
    serialPort1.Write(buff, 0, 1);

    // Set the KeyPress event as handled so the character won't
    // display locally. If you want it to display, omit the next line.

    e.Handled = true;
}

private void btnStop_Click(object sender, EventArgs e)
{
    if (serialPort1.IsOpen)
    {
        serialPort1.Close();
        btnStart.Enabled = true;
        btnStop.Enabled = false;
        textBox1.ReadOnly = true;
    }
}

private void frmMain_FormClosing(object sender, FormClosingEventArgs e)
{
    if (serialPort1.IsOpen) serialPort1.Close();
}