@m_timmy , I found what was missing in the serial port configuration. Last night, I was testing with different terminals—some worked fine, while others didn't. However, I couldn't pinpoint the difference between their settings. In the end, I created my own extremely basic terminal to figure out what was happening. So, I tried various configurations and discovered that the missing piece was activating the "DTR" signal on the computer's side of the port, and that's it:
@UKHeliBob, By default, PuTTY has flow control enabled:
On the other hand, what we were trying to figure out is why the USB serial works fine in the serial monitor but didn't behave as expected in other terminals. Part of the reason seems to be that, since it's not a UART port, it must have the complete signals of an RS-232, unlike pins 0 and 1, which are just two signals without flow control or anything like that.
My conclusions are:
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The USB serial of the UNO R4 has a complete serial port, not just TX and RX.
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f you want to use the USB serial of a UNO R4, you need to enable the "DTR" signal for it to work.