void setup()
{
  Serial.begin(2400);
  pinMode(13, OUTPUT);
}

void loop()
{
  // if incoming serial
  if (Serial.available()) {
    readSerial();
    digitalWrite(13, HIGH);
  } 
  else {
    digitalWrite(13, LOW);
  }
  delay(500);
} 
void readSerial() {
  char val = Serial.read();
  Serial.print(val);
}