'1. SerialPort einfügen
'2. Sende-Empfangstest bei einer einzelnen COM-Schnittstelle durch die 
' Verbindung von RD+TD (9p = Pin2+Pin3)
Public Class Form1
    Dim WithEvents Btn As New Button With {.Dock = DockStyle.Bottom, .Text = _
      "Senden", .Height = 30, .Parent = Me}
    Dim Tb As New TextBox With {.Dock = DockStyle.Fill, .Multiline = True, _
    .Parent = Me}
 
    Private Sub Form1_Load(sender As Object, e As System.EventArgs) Handles _
      Me.Load
        SerialPort1.PortName = "COM5"  'Eigenen Portname einsetzen!
        SerialPort1.Open()  'Zum Testen reichen die Standardeinstellungen
    End Sub
 
    Private Sub SerialPort1_DataReceived(sender As Object, e As _
      System.IO.Ports.SerialDataReceivedEventArgs) Handles _
      SerialPort1.DataReceived
        Dim daten As String = ""
        Do While SerialPort1.BytesToRead > 0
            daten &= SerialPort1.ReadByte.ToString & ","
        Loop
        Me.Invoke(Sub() Tb.Text = daten) 'Invoke nötig, wenn ein Control aus 
        ' einem anderen Thread angesprochen wird!
    End Sub
 
    Private Sub Btn_Click(sender As Object, ByVal e As EventArgs) Handles _
      Btn.Click
        'Dim txt As String = Chr(0) & "123" & Chr(129)
        'Dim b() As Byte = System.Text.Encoding.Default.GetBytes(txt)
        Dim b() As Byte = {0, 48, 49, 50, 129}
        SerialPort1.Write(b, 0, b.Length)   '0,48,49,50,129
    End Sub
End Class