Just a follow up for my update problem.
=>After many hours spent, with the help of @Shannon
, my problem is a hardware problem.
After reading the CH340E datasheet, the 1rst thing I have done is soldering a Led on pin 6 ("TNOW") which is not used inside, and is a pin to see is there is transmission activity.
After that, I have seen the Led blinking: so no problem with computer or com port or anything else.
So I have investigated more, and have desoldered some compenents to find the culprit.
=>The USB/RS232 transceiver chip (the CH340E) is damaged (shorted) only in one pin: the pin 9 which is the RX (connected to the TX pin of the main chip). The mainchip replies but the CH340E, with its RX pin shorted, doesn't send anything to the USB port.
Hope I'm the only unlucky guy who has this hardware problem Here are some photos I have made if interested.
Shannon will send me the chip