Dim docname As String = "d:\Log.txt"
 
  Dim logWriter As New System.IO.StreamWriter(docname, True)
 
  logWriter.Write(receivedtext)				' von UART oder
  logWriter.Write(rtbreceived.text)			' aus Rich text box
  logWriter.Flush()
  logWriter.Close()

   ..
    Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As _
      System.IO.Ports.SerialDataReceivedEventArgs) Handles _
      SerialPort1.DataReceived
        ReceivedText(SerialPort1.ReadExisting())
    End Sub
 
 
 
    Private Sub ReceivedText(ByVal [text] As String)
        Dim DateTime As String = Now.ToString("dd-MM-yyyy HH_mm_ss")
        Dim docname As String = "Log.txt"
        Dim dt = DateTime                                 ' Datum und Zeit 
        ' ermitteln
 
        Dim answer As String
 
        If rtbreceived.InvokeRequired Then
            Dim x As New SetTextCallback(AddressOf ReceivedText)
            Invoke(x, New Object() {(text)})
        Else
            answer = [text]
            If text_sent <> "RST" Then                                      _
              ' gesendete Abfrage war <> RST
                Try
                    stopflag = True                                             _
                      ' weiteren Datenverkehr stoppen
 
                    rtbreceived.Text &= [text]                                  _
                       ' in Rich text box eintragen
 
                    rtbreceived.SelectionStart = rtbreceived.Text.Length        _
                      ' Autoscroll
                    rtbreceived.ScrollToCaret()
                    rtbreceived.Refresh()
 
                    If text_sent = "SE" Then                                    _
                      ' SE > Datei soll abgerufen und gespeichert werden
                        docname = docname & dt & ".txt"
                        SaveFileDialog1.FileName = docname
                        SaveFileDialog1.InitialDirectory = "d:\"
                        SaveFileDialog1.RestoreDirectory = True
                        SaveFileDialog1.Filter = "Text Files (*.txt*)|*.txt"
                        If SaveFileDialog1.ShowDialog = _
                          Windows.Forms.DialogResult.OK Then
                            My.Computer.FileSystem.WriteAllText( _
                            SaveFileDialog1.FileName, [text], True)    'oder
                            '       My.Computer.FileSystem.WriteAllText( 
                            ' SaveFileDialog1.FileName, rtbreceived.Text, True)
                        End If
                        text_sent = ""
                    End If
                Catch ex As Exception
                    '   MsgBox("Error")
                End Try
 
            End If
 
            text_sent = ""
            stopflag = False                            'Datenverkehr wieder 
            ' zulassen
        End If
 
        '        parse_answer(answer)                       'Auswertung _
          weiterer Befehle
 
 
    End Sub

Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As _
      System.IO.Ports.SerialDataReceivedEventArgs) Handles _
      SerialPort1.DataReceived
        ReceivedText(SerialPort1.ReadExisting())
    End Sub