sniffer_test.ino

String tmp; 

void setup() {
  Serial.begin(115200);
  Serial1.begin(115200); 
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }
  Serial.println("hello!");
};

void loop() {

  while (Serial1.available() > 0)  {
    tmp += char(Serial1.read());
    delay(2);
  }

  if(tmp.length() > 0) {
    Serial.println(tmp);
    tmp = "";
  }

  if (Serial.available()) {
    Serial1.write(Serial.read());
  }
  
  delayMicroseconds(10);
}