Opening and Initializing a 16550 UART-Compatible Interface
When Serial is used as a lower-level device filter driver, the following considerations apply to opening and initializing the filter device:
Serial supports only one open at a time on the filter device.
The filter device is in an undefined state when it is open.
A client should initialize the filter device to a known state before using it. A user-mode client can use the IOCTL_SERIAL_SET_Xxx requests. Note, however, that a Win32-compliant application must use the communication functions that are supported by the Windows Base Services in the Microsoft Windows SDK. A kernel-mode client can use the IOCTL_SERIAL_Xxx and the IOCTL_SERIAL_INTERNAL_Xxx requests.