'Test ohne Gerät! Nur durch die Verbindung von RD+TD (9polig = Pin2 mit Pin3)
Imports System.IO.Ports
 
Public Class Form1
    Dim Tb1 As New TextBox With {.Text = "4387000001", .Dock = DockStyle.Top, _
      .Parent = Me}
    Dim Tb2 As New TextBox With {.Dock = DockStyle.Bottom, .Parent = Me}
    Dim WithEvents Btn As New Button With {.Text = "Senden", .Dock = _
      DockStyle.Bottom, .Parent = Me}
    'PortName einsetzen und Abweichung von der Standardeinstellung angeben
    Dim WithEvents myComPort As New SerialPort("COM9") With {.StopBits = _
      StopBits.Two}
 
    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) _
      Handles MyBase.Load
        myComPort.Open()
    End Sub
 
    Private Sub Btn_Click(sender As Object, e As EventArgs) Handles Btn.Click
        If Tb1.Text.Count = 10 Then ' 5 Hex-Zeichen-Befehle!
            WriteHexString(Tb1.Text)
        End If
    End Sub
 
    Private Sub WriteHexString(txt As String)
        Dim b(4) As Byte
        Dim nr As Int32
        For i As Int32 = 0 To txt.Length - 1 Step 2
            b(nr) = Convert.ToByte(txt.Substring(i, 2), 16)
            nr += 1
        Next
        myComPort.Write(b, 0, b.Count)
    End Sub
 
    Private Sub myComPort_DataReceived(sender As Object, e As _
      SerialDataReceivedEventArgs) Handles myComPort.DataReceived
        Me.Invoke(Sub()
                      Dim txt As String = ""
                      Do While myComPort.BytesToRead > 0
                          txt &= "  " & myComPort.ReadByte
                      Loop
                      Tb2.Text = txt ' Bytes in Tb2 ausgeben
                  End Sub)
    End Sub
End Class