void setup() {
  Serial.begin(115200);
  Serial.println("Ready");
}

String readSerialData() {
  String result = "";
  if(Serial.available()) {
    while (Serial.available()) {
      char input = Serial.read();
      Serial.print(input); 
      result.concat(input);
      delay(3); 
    }
    Serial.println();
    Serial.print("Result: '");
    Serial.print(result);
    Serial.print("' (");
    Serial.print(result.length());
    Serial.println(" char)");
  }
  else {
    //Serial.println("No Data Available");
  }
  return result;
}


void loop() {
  String receivedData = readSerialData();
  if (receivedData.length() > 0) { // Ici je ne rentre jamais car receivedData est toujours vide
    Serial.print("Scan reçu: '");
    Serial.print(receivedData);
    Serial.print("' (");
    Serial.print(receivedData.length());
    Serial.println(" char)");
  }
}