void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

void loop() {
  // send data only when you receive data:
  while (Serial.available() > 0) {
    // read the incoming byte:
    int incomingByte = Serial.read();

    Serial.write(incomingByte);
  }

  static int no = 0;
  
  Serial.print("serial port test ");
  Serial.println(no++);
  delay(2000);
}