How to write MPI/USB driver for S7 mpi/usb adapter?

Does anyone has written usb driver for S7 mpi/usb adapter(6ES7972 0CB20 0XA0).We want to do this.(OS:windoexp,VC6.0++)
We have found the adapter device (VID_0908 & PID_0004)using general usb driver.We then Createfile()successfully.We send(call function writefile()) the mpi first byte(0x02) to PLC,then start a read thread to read msg (call function readfile()),PC came to bluescreen,read failed.

Any comments for me? Appreciate any help.

It's ok

The ReadFile() problem has been solved by changing the receiving or sending buffer.My COM default size is2048.When I set 256,the driver runs well.So I can only set the datablock address length to be 210.
The speed of communication is 100ms/210 bytes by MPI/USB cable.
As a doubt, Why I can't set the buffer size to be 2048?
Thanks anyone anyway.

Hi
What`s part number ic ( chip )usb controller of simatic usb adapter?