Serial Communication DUE = serial Monitor works Serial Converter does not work

Good evening everyone,

hope you can give me some hints (or a solution:-) ) for my problem with Arduino DUE and Serial Communication.

AS Hardware I have the DUE, some LEDS i want to switch on/off and a TTL-RS232 Converter (with MAX232).

My Arduino Code works, via Serial Monitor I can switch on/off my Leds.

BUT: Using the Coverter and crossed RS232 Cable (and RS232 USB Converter) I get no response, no difference if I send Data from DUE or via Terminal.

Baudrate, Parity, … should be set up correctly.

I can see a TX LED flashing on the converter, so I think the communication from DUE to Serial Converter is fine…

void setup()
{
  // start serial port at 9600 bps:
//  Serial1.begin(115200);
    Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }

establishContact();  // send a byte to establish contact until receiver responds

  pinMode(A_green, OUTPUT);
  pinMode(A_blue, OUTPUT);
  pinMode(A_red, OUTPUT);
  pinMode(B_green, OUTPUT);
  pinMode(B_blue, OUTPUT);
  pinMode(B_red, OUTPUT);
  
  digitalWrite(A_green, HIGH);
  digitalWrite(A_blue, HIGH);
  digitalWrite(A_red, HIGH);
  digitalWrite(B_green, HIGH);
  digitalWrite(B_blue, HIGH);
  digitalWrite(B_red, HIGH);

}


// Void Loop is here, code not necessary to be posted.....

void establishContact() {
  while (Serial.available() <= 0) {
    Serial.println('A');   // send a capital A
    delay(1000);
  }
}

Does anyone have an idea why I can communicate via serial Monitor with the DUE but NOT via Converter and Terminal?

many thanks for every hint!

Wolfgang

Update: seems to be the serial converter, fumbling with an oscilloscope to it it seems that no signal is passing the max232....