I have not seen this phenomenon.
Could you please explain your settings?
Does it happen even if you use Default Configuration setting?
Yes, I am using the default setting.
And the serial port configuration is:
baud rate: 19200
data bit: 8
stop bit: 1
hand shake: hardware
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Sorry for the delay in my response.
USB bus will enter idle mode if the COM port is closed and USB host can selective suspend the device after idle timeout.
This selective suspend will put the USB-UART device to deep sleep with Tx pin held Hi-Z mode.
Tx pin not in Hi-Z will violate USB specification VBUS current consumption limits.
Thank you for your reply.
I confirmed that TxD remains High when disabling selective suspend.