'RTS und CTS über ein Kabel, Schalter oder Taster verbinden.
'Der Zustand wird entsprechend angezeigt.
Imports System.IO.Ports
Public Class Form1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        SerialPort1.RtsEnable = True
        SerialPort1.Open()
    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 Me.Invalidate( _
        )
    End Sub
 
    Private Sub Form1_Paint(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.PaintEventArgs) Handles Me.Paint
        Using p As New Pen(Color.Black, 8) With {.EndCap = _
        Drawing2D.LineCap.Round}
            e.Graphics.SmoothingMode = Drawing2D.SmoothingMode.AntiAlias
            e.Graphics.FillEllipse(Brushes.Black, 100, 130, 20, 20)
            e.Graphics.FillEllipse(Brushes.Black, 180, 130, 20, 20)
            Select Case SerialPort1.CtsHolding
                Case True : e.Graphics.DrawLine(p, 115, 140, 185, 140)
                Case False : e.Graphics.DrawLine(p, 115, 138, 185, 100)
            End Select
        End Using
    End Sub
End Class