int pin = LED_BUILTIN;
bool isOn = false;
void setup() {
  Serial.begin(9600);
  pinMode(pin, OUTPUT);
}

void sendUpdate() {
      if (isOn) {
        Serial.write(0x00);
        Serial.write(0xFF);
      }
      else {
        Serial.write(0x00);
        Serial.write(0x00);
      }
}

void loop() {
  if (Serial.available() >= 2) {
    int a = Serial.read();
    int b = Serial.read();
    if (a == 0xFF && b == 0xFF) {
      digitalWrite(pin, HIGH);
      isOn = true;
      sendUpdate();
    }
    else if (a == 0xFF && b == 0x00) {
      digitalWrite(pin, LOW);
      isOn = false;
      sendUpdate();
    }
    else if (a == 0xFF && b == 0x01) {
      sendUpdate();
    }
  }
  delay(10);
}