Hi,
I encounter a specific problem when using the functions from D2XX.DLL to communicate to the FT232BM USB->Serial chip.
I use in my programs a SubVI which passes all the communication to and from the USB chip, 'USB_Send_Receive.vi'. In that VI first a dll function 'Open' is called, which returns a Handle used in subsequent Read and Write calls, and at the end of the VI the dll function 'Close' is called which releases the Handle. That all works fine.
The problem is that frequent calls of the 'Open'->'Read/Write'->'Close' sequence slow down my main app appreciably. (The slowing doen is by a factor 10 in my top level measurement program). Since the repeated Open/Close is not necessary in principle, I changed the 'USB_Send_Receive.vi' to perform an 'Open' only on first call and store the Handle in an uninitialized shift register. No 'Close' is perfomed. That also works fine, but only as long as the VI is kept in memory. When it is released from memory, later reloaded and started again, the VI will call the 'Open' function from the dll which now returns an error,(probably since the last handle has not been cleared by a 'Close' call). That does not happen when LabVIEW itself is released from memory, so obviously releasing LabVIEW does more cleanup with the used dll than just releasing the VI using the dll.
I don't know how to proceed here and try to find help in this forum.
What I tried already is this:
I stored the handle (which is a 32 bit integer) aquired by a successful 'Open' call in a file and reused this value later after releasing, reloading, restarting the VI. That also failed (knowing the handle obviously is not the same as 'owning' the handle).
What I will try next is to call the 'USB_Send_Receive.vi' with some flag telling it to release the handle when it is no longer used. I don't like that solution, though, since it shifts the responsibility of rleasing the handle from the VI which aquires it to a higher level VI.
What I ideally would like to have is a sort of 'On Exit' function in Labview, which is called automatically when a VI is removed from memory (i.e. the counterpart of an automatic initialize code). AFAIK this is not implemented yet, so I'll put it on the LV feature wishlist for the future...
Franz
