Dim wait As Boolean
Dim buf As Byte

Ser2.BaseStream.WriteByte(100)              ' 1.Meßbefehl
     wait = True                                          
     Do While Not wait                        ' auf das Ergebnis warten
       Application.DoEvents()
      Loop
Ser2.BaseStream.WriteByte(50)               '2.Meßbefehl

Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As _
  System.IO.Ports.SerialDataReceivedEventArgs) Handles Ser1.DataReceived
        Dim b As Byte = Ser1.BaseStream.ReadByte
        Ser1.BaseStream.WriteByte(b + b)
End Sub

Private Sub Ser2_DataReceived(ByVal sender As Object, ByVal e As _
  System.IO.Ports.SerialDataReceivedEventArgs) Handles Ser2.DataReceived
        buf = Ser2.BaseStream.ReadByte
        Me.Invoke(Datenanzeige)                   ' Anzeige aufrufen
End Sub
Private Sub Anzeige()
        LBox.Items.Add(buf.ToString)              ' berechneter Wert in der 
        ' ListBox anzeigen
        wait = False                               ' neue Messung freigeben
End Sub