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
...