Public Class Form1
    Dim sw As New Stopwatch
    Dim run As Boolean
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        SerialPort1.Open()
        Timer1.Start()
    End Sub
 
    Private Sub SerialPort1_PinChanged(ByVal sender As Object, ByVal e As _
      System.IO.Ports.SerialPinChangedEventArgs) Handles SerialPort1.PinChanged
        If e.EventType = IO.Ports.SerialPinChange.CtsChanged Then
            If SerialPort1.CtsHolding AndAlso Not run Then
                run = True
                sw.Reset()
                sw.Start()
            ElseIf SerialPort1.CtsHolding AndAlso run Then
                run = False
                sw.Stop()
            End If
        End If
    End Sub
 
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Timer1.Tick
        Me.Text = sw.Elapsed.ToString
    End Sub
End Class