// https://forum.arduino.cc/t/auslesen-der-easymeter-q3m-d0-schnittstelle/548183

int invertirpin = 6;

void setup() {
  Serial.begin(9600);
  pinMode(invertirpin,OUTPUT);
  digitalWrite(invertirpin,HIGH);  // HIGH non inv; low inv  74LS02 als xor vor Reciver-Pin  
}

byte incomingByte;

String sMessage = "";

void loop() {
  while (Serial.available() > 0) {
    incomingByte = Serial.read();
    if (incomingByte < 0x10)
      sMessage += "0";

    sMessage += String(incomingByte, HEX);
    sMessage += " ";
  }

  if(sMessage != ""){
    Serial.println(sMessage);
    sMessage = "";
  }
}