' Flip the bit 'Bit' in byte 'Byte' and return the new byte.
function BitFlip(Byte, Bit)
    if  TstBit(Byte, Bit) = 1 then
        BitFlip = (Byte AND (255 XOR (2^Bit)))
    else
        BitFlip = (Byte OR (2^Bit))
    end if
end function

' Test bit "Bit" in variable "Byte" and return it's value (1 or 0)
function TstBit(Byte, Bit)
    TestBit = 0
    if ((Byte AND (2^Bit)) > 0) then
        TstBit = 1
    end if
end function

' This code section goes at the very top of your LB program
    Global  kMaxCOMPorts            ' Maximum number (COMxx) and largest number _of_ ports to store
    kMaxCOMPorts = 40
    DIM CombComPort$(kMaxCOMPorts)  ' Holds strings for COM port numbers (COM1, COM2, etc.)    

' This code section goes in your GUI definition - before you open the window that contains the pull down list.
' The pull down list (Combo Box) is called CombComPort, and the array that fills it is called CombComPort$
    combobox #main.CombComPort, CombComPort$(), [CombComPortDoubleClick],   75, 372, 100, 100

' This code section goes in the code that you use to intialize the stuff in the window.
' It does the work of populating the drop down list if all available COM ports
' Populate the drop down list of available COM ports
    gosub [GetCOMPorts]

' This code section is what gets called when the user pulls down the combot box and selects one of the items
[CombComPortDoubleClick]   
    ' Perform action for the combobox named 'CombComPort'
    ' You should probably do other things here like close any other open ports, etc.
    print #main.CombComPort, "contents? ComPort$"
    open ComPort$;":9600,8,N,1,RS,DS0,CS0" for random as #commHandle
    wait

' This section of code is the real meat of this example. This subroutine and function do the hard
' work of figuring out which com ports are available.
' This subroutine will populate the drop-down list called #main.CombComPort
' with all of the currently available COM ports. You can change #main.CombComPort
' to whatever you use in your program.
[GetCOMPorts]
    ' First clear out #main.CombComPort
    for Port = 0 to kMaxCOMPorts
        CombComPort$(Port) = ""
    next Port

    CurrentComboIndex = 0
    
    ' We're going to loop from 1 to MaxCOMPorts and see if each port is available
    for TestPort = 1 to kMaxCOMPorts
        if IsCOMPortAvailable(TestPort) = 1 then
            ' Add this port into #main.CombComPort
            CombComPort$(CurrentComboIndex) = "COM" + str$(TestPort)
            CurrentComboIndex = CurrentComboIndex + 1
        end if
    next TestPort

    ' Now reload the combbox to get the new array values
    print #main.CombComPort, "reload"

    return

' Helper function for [GetCOMPorts]
' Returns 0 if the Port is not available on this system (either already opened or doesn't
' exist) and 1 if the port is available.
function IsCOMPortAvailable(Port)
    struct lpSecurityAttributes, _
        nLength as long, _
        lpSecurityDescriptor as long, _
        bInheritHandle as long

    lpFileName$ = "\\.\COM" + str$(Port)

    dwDesiredAccess = 0
    dwShareMode = 1 + 2
    dwCreationDisposition = 3
    dwFlagsAndAttributes = 128
    hTemplateFile = 0

    calldll #kernel32, "CreateFileA", _
        lpFileName$ as ptr, _
        dwDesiredAccess as long, _
        dwShareMode as long, _
        lpSecurityAttributes as struct, _
        dwCreationDisposition as long, _
        dwFlagsAndAttributes as long, _
        hTemplateFile as long, _
        retcode as long
    hObject = retcode

    ' Test the return code to see if the port is available and free
    if retcode = -1 then
        IsCOMPortAvailable = 0
    else
        ' If we were able to open it, then close it and return TRUE
        calldll #kernel32, "CloseHandle", _
            hObject as long, _
            retcode as long

        IsCOMPortAvailable = 1
    end if
end function

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