Virtual COM port emulates legacy COM port on PC. A device driver on PC shows USB ports as usual COM port. For Windows, it means that the port is accessed using these Win-API.
"Communications Functions" from MSDN
http://msdn2.microsoft.com/en-us/lib...63194.aspx

And USB CDC spec just defines how to carry RS232 signaling over USB.
This table shows the relation between Win-API, RS232 signaling, USB CDC spec and UART on the chip.

Win APIs                RS232                 USB                UART
SetCommState()       - Setting parameters   - Endpoint 0       - command register
EscapeCommFunction()   (baud, data bits..)                       status register
SetCommBreak()         DTR, RTS
ClearCommBreak()

GetCommState()       - DSR, DCD, RI         - interrupt IN EP  - status register
ClearCommError()       Break status
GetCommModemStatus()   Parity error
                       Frame error
                       Overrun error
                   
WriteFile()          - TX                   - bulk OUT EP      - UART TX

ReadFile()           - RX                   - bulk IN EP       - UART RX

 Host application <--> Device driver <-----> USB hardware <--> Firmware

You may be aware of it, USB CDC spec doesn't support CTS.


There is no reason that we have to always apply USB CDC spec to carry RS232 signaling. We can define the method to carry it over USB freely. It is called as direct port.

In this case, another device driver is required.
For example, the direct port is realized using a generic bulk device driver, such as usb_bulk example from Win DDK (downloaded free from MS).

"DDK - Windows Driver Development Kit"
http://www.microsoft.com/whdc/DevT...fault.mspx

Direct port example

Win APIs                RS232                 USB                UART
WriteFile()          - Setting parameters   - bulk OUT EP1     - command register
ReadFile()             Handshake              bulk IN EP1        status register
                       Error status

WriteFile()          - TX                   - bulk OUT EP2     - UART TX
ReadFile()           - RX                   - bulk IN EP2      - UART RX

Direct port is shown to application as USB ports.

When we apply direct port, the signals of multiple UARTs are assigned to it easily.

Win APIs                RS232                 USB                UARTs
(multiplexed on UARTs)
WriteFile()          - Setting parameters   - bulk OUT EP1     - command registers
ReadFile()             Handshake              bulk IN EP1        status registers
                       Error status

WriteFile()          - TX                   - bulk OUT EP2     - UART1 TX
ReadFile()           - RX                   - bulk IN EP2      - UART1 RX

WriteFile()          - TX                   - bulk OUT EP3     - UART2 TX
ReadFile()           - RX                   - bulk IN EP3      - UART2 RX
...

Tsuneo