Serial comm between USB communication controller and main controller on Uno

Hi there,

on the Arduino, there is a communication controller (an ATmega8u I think) and a main controller (an ATmega328). Both are connected: TX of comm to RX of main, RX of comm to TX of main, both via a 1k resistor.

The firmware of the comm controller enables the TXEN flag and configures TX as output by that in the initialization phase and does not reset it. This means that the RX line of the main controller is driven all time, via the resistor with a signal.

If I want to feed serial communication into the USART of the main controller via the RX/TX pins of it I need to overwrite the signal at TX of the comm controller with considerable current to impress my own TX signal.

However, as soon as USB is unplugged and the receive buffer of the comm controller is completely drained to the main controller, I would imagine, the TX pin of the comm controller could be switched to high state by disabling the TXEN flag, enabling one to connect something else to the RX pin of the main controller. As soon as USB is plugged again, the comm controller could reenable TXEN and drive the RX pin of the main controller using its own TX pin again.

Is there a reason for not doing that in the USB communication firmware of the Arduino Uno?

Cheers, Wolfgang