Application software can then access the remote
ports as if they were local ports.
Low-level drivers on the PC handle hardware-specific details such as detecting
ports, assigning names, and managing communications with applications.
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Several drivers provided with Windows each manage different aspects of
COM-port communications. The serial.sys driver controls communications
with COM-port devices. The serenum.sys driver is an upper-level filter driver
that enumerates and retrieves identifying information from Plug-and-Play
COM-port devices
A USB device accessed as a COM-port device requires an additional driver. The
usbser.sys driver is a bus driver that manages communications between the operating
system??™s COM-port driver and USB drivers. The usbser.sys driver requests
to send and receive COM-port data in USB bulk transfers and implements protocols
for sending and receiving status and control information in USB transfers.
Some USB virtual COM-port devices use vendor-specific drivers in place
of the Windows-provided drivers. Chapter 15 and Chapter 16 have more about
USB virtual COM ports.
Many devices can use the Windows-provided drivers or a driver provided by a
USB-controller manufacturer or another third party. For low-level programming,
Microsoft??™s Windows Driver Kit (WDK) provides a variety of example
code, including source code for the serial and serenum drivers.
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