Use arduino (due) as serial tunnel when your usb 2 serial cable is lost

serialTunnel.ino

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
  Serial1.begin(9600);
  blink();
  blink();
  blink();
}

void blink() {
  digitalWrite(13, HIGH);   
  delay(100);              
  digitalWrite(13, LOW);   
  delay(100);              
}

void loop() {
  
  serialEvent(); //call the function
}


void serialEvent() {
  while (Serial1.available()) {
    int inChar = Serial1.read();
    Serial.write(inChar);
  }
  while (Serial.available()) {
    int inChar = Serial.read();
    Serial1.write(inChar);
  }
}