Private Const ErrMessage As String = "Bitte ein Buchstabe ins Textfeld" & _
  "schreiben."
 
Private Sub Form_Load()
Text1.MaxLength = 1
Text1.Text = "A"
Label1.Caption = ErrMessage
End Sub
 
Private Sub Text1_KeyPress(KeyAscii As Integer)
Dim TmpAscii As Integer, ExitFor As Integer
 
If KeyAscii = 13 Then
  If Len(Text1.Text) = 0 Then
    Label1.Caption = ErrMessage
    GoTo EndSub
  End If
  Label1.Caption = ""
 
  TmpAscii = Asc(Text1.Text)
  If TmpAscii >= 97 And TmpAscii <= 122 Then 'Kleine buchstaben
    ExitFor = 122
  ElseIf TmpAscii >= 65 And TmpAscii <= 90 Then 'Grosse Buchstaben
    ExitFor = 90
  Else
    Label1.Caption = "Kein gültiges zeichen!  "" " & Chr$(TmpAscii) & " """ & _
                     vbCrLf & ErrMessage
    GoTo EndSub
  End If
  Label1.Caption = Chr$(TmpAscii) & " <--- Ihre eingabe" & vbCrLf
  TmpAscii = TmpAscii + 1
  For TmpAscii = TmpAscii To ExitFor
    Label1.Caption = Label1.Caption & Chr$(TmpAscii)
  Next TmpAscii
End If
 
EndSub:
Text1.SetFocus
Text1.SelStart = 0
Text1.SelText = Text1.Text
Text1.SelLength = 1
End Sub