Creating the Chat Application
Using Visual Studio 2005, create a new Windows application and save it as C:\SerialCommChat. Populate the default Form1 as shown in Figure 3.

Figure 3. Controlling: Populate the default Form1 with the various controls, as shown in the figure, including buttons, labels, and text box controls.

Set the properties for the various controls as shown in Table 1.

Table 1. Set the properties for the various controls, as shown.

CONTROL	PROPERTY	VALUE
txtDataReceived	Scrollbars	Vertical
txtDataToSend	Multiline	True

Switch to the Code View for Form1 to start coding the form. First, declare a member variable SerialPort to represent the serial port that you want to work with:

Public Class Form1
    Dim WithEvents serialPort As New IO.Ports.SerialPort

Author's Note: In addition to the SerialPort you can also use the IO.Ports.SerialPort class; both are the same.

Notice that you need to declare SerialPort with the WithEvents keyword. This is because the SerialPort class has the DataReceived event that is fired when data arrives at the serial port and hence you need to service this event to receive the data.

When the form is first loaded, you will retrieve all the available serial port names on your computer using the My.Computer.Ports.SerialPortNames collection and then add these port names to the ComboBox control:

    Private Sub Form1_Load( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles MyBase.Load

        For i As Integer = 0 To _
           My.Computer.Ports.SerialPortNames.Count - 1
            cbbCOMPorts.Items.Add( _
               My.Computer.Ports.SerialPortNames(i))
        Next
        btnDisconnect.Enabled = False
    End Sub

Figure 4 shows what the ComboBox control will look like when the form is first loaded.

Figure 4. Port o' Call: The screen shot shows the ComboBox control displaying all the serial port names.

Once a port name is selected, the user clicks the Connect button to open the selected port. This is accomplished by the following method:

    '-------------------------------------------
    ' Event handler for the Connect button
    '-------------------------------------------
    Private Sub btnConnect_Click( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles btnConnect.Click

        If serialPort.IsOpen Then
            serialPort.Close()
        End If

        Try
            With serialPort
                .PortName = cbbCOMPorts.Text
                .BaudRate = 9600
                .Parity = IO.Ports.Parity.None
                .DataBits = 8
                .StopBits = IO.Ports.StopBits.One
            End With
            serialPort.Open()

            lblMessage.Text = cbbCOMPorts.Text & " connected."
            btnConnect.Enabled = False
            btnDisconnect.Enabled = True
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

In particular, notice that I have explicitly set the various properties of the SerialPort class, such as PortName, BaudRate, Parity, etc. These are the communications parameters you need to set when communicating with serial devices. When communicating with serial devices, check their settings such as baudrate, parity, databits, and stopbits and set them accordingly in your application.

The Disconnect button closes the current opened serial port:

    '-------------------------------------------
    ' Event handler for the Disconnect button
    '-------------------------------------------
    Private Sub btnDisconnect_Click( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles btnDisconnect.Click
        Try
            serialPort.Close()
            lblMessage.Text = serialPort.PortName & " disconnected."
            btnConnect.Enabled = True
            btnDisconnect.Enabled = False
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

To send data to the recipient through the serial port, use the Write() method of the SerialPort class:

    '-------------------------------------------
    ' Event handler for the Send button
    '-------------------------------------------
    Private Sub btnSend_Click( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles btnSend.Click
        Try
            serialPort.Write(txtDataToSend.Text & vbCrLf)
            With txtDataReceived
                .SelectionColor = Color.Black
                .AppendText(txtDataToSend.Text & vbCrLf)
                .ScrollToCaret()
            End With
            txtDataToSend.Text = String.Empty
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

One nice feature of the SerialPort class is that you need not constantly poll for incoming data. Instead, you just need to service the DataReceived event and it will automatically fire when incoming data is detected. However, as this event is running on a separate thread, any attempt to update the main Form directly will result in an error. Hence, you need to use a delegate to update controls on the main thread (Form1):

    '-------------------------------------------
    ' Event handler for the DataReceived
    '-------------------------------------------
    Private Sub DataReceived( _
       ByVal sender As Object, _
       ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) _
       Handles serialPort.DataReceived

        txtDataReceived.Invoke(New _
                       myDelegate(AddressOf updateTextBox), _
                       New Object() {})
    End Sub

The delegate and the updateTextBox() subroutine is defined as follows:

    '------------------------------------------------------
    ' Delegate and subroutine to update the Textbox control
    '------------------------------------------------------
    Public Delegate Sub myDelegate()
    Public Sub updateTextBox()
        With txtDataReceived
            .Font = New Font("Garamond", 12.0!, FontStyle.Bold)
            .SelectionColor = Color.Red
            .AppendText(serialPort.ReadExisting)
            .ScrollToCaret()
        End With
    End Sub