' Change one bit in the control register.
Private Sub ControlCheck_Click ( Index As Integer )
  If Self Then Exit Sub
  Dim register As Byte
  register = Inp ( PortAddress + 2 )
  register = register Xor 2 ^ Index
  Out PortAddress + 2, register
End Sub

' Change one bit in the data register.
Private Sub DataCheck_Click ( Index As Integer )
  If Self Then Exit Sub
  If ControlCheck ( 5 ).Value = 1 Then Beep
  Dim register As Byte
  register = Inp ( PortAddress )
  register = register Xor 2 ^ Index
  Out PortAddress, register
End Sub

' Changes to the feedback register are impossible.
Private Sub FeedCheck_Click ( Index As Integer )
  If Not Self Then Beep
End Sub

' Start on parallel port #1.
Private Sub Form_Load ()
  PortSelect(0).Value = True
End Sub

' Scan all three registers and update the display.
' Repeat ten times a second.
Private Sub Poll_Timer ()
  Dim register As Byte
  Dim i As Byte
  Self = True ' avoid recursive changes
  ' Read data register
  register = Inp ( PortAddress )
  For i = 0 To 7
    If register And 2 ^ i Then
      DataCheck ( i ).Value = 1
    Else
      DataCheck ( i ).Value = 0
    End If
  Next i
  ' Read feedback register
  register = Inp ( PortAddress + 1 )
  For i = 3 To 7
    If register And 2 ^ i Then
      FeedCheck ( i ).Value = 1
    Else
      FeedCheck ( i ).Value = 0
    End If
  Next i
  ' Read control register
  register = Inp ( PortAddress + 2 )
  For i = 0 To 5
    If register And 2 ^ i Then
      ControlCheck ( i ).Value = 1
    Else
      ControlCheck ( i ).Value = 0
    End If
  Next i
  Self = False
End Sub

' Change the parallel port.
Private Sub PortSelect_Click ( Index As Integer )
  Select Case Index
  Case 0
    PortAddress = &H3BC
  Case 1
    PortAddress = &H378
  Case 2
    PortAddress = &H278
  End Select
End Sub