[Mega2560] Using serial 1/2/3 ?

Hello

I'm trying using Serial 1/2/3 of an arduino Mega . I followed the first example of the thread "Serial input Basics ".
This works fine with serial (so connected via USb Arduino native driver ) but I'm unable to make it working (I don't receive caracters ) with the the others serial (1/2/3) and i don't understand why. Can the converter usb/RS232 be faultive ? I used this one :
https://www.ugreen.com/product/UGREEN_USB_to_RS232_DB9_Serial_Cable_Male_A_Converter_Adapter-en.html .
Is it some recommanded USB hardware / drivers to use without problems the serials 1/2/3

Thanks a lot for your help .

Sorry, my crystal ball is broken. Please post your code AND a schematic. Please don't post a Fritzing breadboard mess, a simple hand drawing will do. See How to use the forum for more info.

and what did you connect to SerialX?

the code I use is the code of the first example of thread "Serial Input Basics - updated"

// Example 1 - Receiving single characters

char receivedChar;
boolean newData = false;

void setup() {
    Serial.begin(9600);
    Serial.println("<Arduino is ready>");
}

void loop() {
    recvOneChar();
    showNewData();
}

void recvOneChar() {
    if (Serial.available() > 0) {
        receivedChar = Serial.read();
        newData = true;
    }
}

void showNewData() {
    if (newData == true) {
        Serial.print("This just in ... ");
        Serial.println(receivedChar);
        newData = false;
    }
}

I just changed "serial" for "serial1" ( or 2/3) . on the other side , I use an PC with just a Serial port monitor (Eltima Software).
With the converter Ugreen ,I use a passive interface (passive connection rs232 )

So I have juste three wires 2/3 /5 directly on the TX1/RX1/ GND (VS tx2/rx2/GND or rx3/rx3/gnd) of the Mega

So what is connected to that Serial? And what is Eltima Software??????

Do note, the Arduino does NOT output RS232 and can be damaged by it.

You should get a USB to TTL serial adapter. They are available from all the usual sources (eBay, Aliexpress, Amazon, SparkFun, Adafruit, etc.) for a low price and are a very useful tool to have on hand. The most common are the ones based on the FT232 or the CH340. Either will work fine.

septillion:
Do note, the Arduino does NOT output RS232 and can be damaged by it.

?

RS232 uses wayyy higher voltages, up to plus and minus 25V! Arduino can only work with 5V (and NO -5V).