Description of the problem in the MSDN: https://msdn.microsoft.com/ru-ru/en-en/library/aa363858(v=vs.85).aspx "To specify a COM port number greater than 9, use the following syntax: "\.\COM10". This syntax works for all port numbers and hardware that allows COM port numbers to be specified." But there is not quite right :) They write "\.\COM10" it's not going to work. Correctly so: "\\.\COM10"

I tested on Win7x32, COM > 9. Work correctly.