'Sende-Empfangstest bei einer einzelnen COM-Schnittstelle
'durch die Verbindung von RD+TD (9polig = Pin2 mit Pin3)
Public Class Form1
    Dim WithEvents Btn As New Button With {.Dock = DockStyle.Bottom, .Text = _
      "Senden", .Height = 30, .Parent = Me}
    Dim WithEvents Tb1 As New TextBox With {.Dock = DockStyle.Left, .Multiline _
    = True, .Parent = Me}
    Dim WithEvents Tb2 As New TextBox With {.Dock = DockStyle.Right, .Multiline _
    = True, .Parent = Me}
    Dim WithEvents T As New Timer
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Me.Load
        SerialPort1.PortName = "COM5"   'Eigenen Portname einsetzen
        SerialPort1.Open()
        T.Start()
    End Sub
 
    Private Sub Btn_MouseDown(sender As Object, e As EventArgs) Handles _
      Btn.MouseDown
        SerialPort1.WriteLine("Hallo")
    End Sub
 
    Private Sub T_Tick(sender As Object, e As System.EventArgs) Handles T.Tick
        Dim b As Byte
        Do While SerialPort1.BytesToRead > 0
            b = CByte(SerialPort1.ReadByte)
            Tb1.Text &= "  " & b
            Tb2.Text &= Chr(b)
        Loop
    End Sub
End Class