void setup() {
  Serial.begin(9600); Serial.println("\n mega-serial-steuern.ino");
  Serial1.begin(115200); Serial1.println("\n mega-serial-steuern.ino");
}

unsigned int cnt=1;
void loop() {
  while (Serial.available()) {
    Serial1.println(Serial.read());
  }
  Serial1.println(String(cnt)+". 'Hi' from MEGA");
  while (Serial1.available()) {
    Serial.println(Serial1.read());
  }
  Serial.print("."); delay(500); cnt++;
}