The program opens the port for output using an Open statement. If the port is in use, it gets error 75 (path/file access).

The intent of this program is to see if you are logged on to the network. It doesn't do that if you do not log on via a COM port. For example, my computer has a DSL modem that is not a COM port so this method cannot tell when I am logged on.

Private Sub Command1_Click()
Dim fnum As Integer
Dim port_in_use As Boolean

    fnum = FreeFile
    On Error Resume Next
    Open Text1.Text For Output As #fnum
    port_in_use = (Err.Number = 75)
    On Error GoTo 0

    If port_in_use Then
        MsgBox "Port in use"
    Else
        MsgBox "Port not in use"
    End If
End Sub