TM4C129ENCPDT: Flow control on USB serial port, DSR/DTR does not work properly

Hi Muneto-san,

  I'm going through the TivaWare usb_dev_cdcserial.c example and there is comment that says RTS/DTR are not supported. 

//****************************************************************************
//
// Note:
//
// This example is intended to run on TM4C evaluation kit hardware
// where the UARTs are wired solely for TX and RX, and do not have GPIOs
// connected to act as handshake signals. As a result, this example mimics
// the case where communication is always possible. It reports DSR, DCD
// and CTS as high to ensure that the USB host recognizes that data can be
// sent and merely ignores the host's requested DTR and RTS states. "TODO"
// comments in the code indicate where code would be required to add support
// for real handshakes.

//****************************************************************************
//
// Set the state of the RS232 RTS and DTR signals. Handshaking is not
// supported so this request will be ignored.
//
//****************************************************************************
static void
SetControlLineState(unsigned short usState)
{
}

Reading this post further, it looks like you would need to modify the USB library in order to support the flow control.

https://e2e.ti.com/support/microcontrollers/arm-based-microcontrollers-group/arm-based-microcontrollers/f/arm-based-microcontrollers-forum/841327/tm4c129xnczad-usb-cdc-device-class-specific-request-to-read-the-virtual-handshaking-signals-cts-dsr-ri-dcd?tisearch=e2e-sitesearch&keymatch=dsr#