There is an interesting issue that may appear when you try to communicate with FT232R virtual serial port using WinAPI. The problem probably persists for other access methods too.

Problem description: - The function WriteFile reports unusually large transfers (e.g. 4104 bytes instead of 1 byte) - It reports 'success', and GetLastError() shows nothing - Actually it sends the amount of data reported, or instead skips sending completely

If you experience the problems mentioned above - ask yourself one simple question. Haven't you connected the FTDI chip to the USB 3.0 port ?

Yes - in my case connecting it to USB 2.0 solved the problem.

(Observed on: Samsung RF711, Windows 7 64-bit, FT232RL)