Nano sends via serial without power connected

Hello everyone,

please note: I am able to send and receive data from an Arduino Nano to a Mega. My issue is that the Nano is responding even when only the Rx/Tx lines are connected, but not power.

This bothers me because in the Mega I have a connection watchdog that sends a byte to the Nano and checks if it is bounced back. Now with power disconnected on the Nano I want the watchdog to tell me that the Nano is not connected. But it happily sends back the byte everytime!

Ports used on the Mega: Serial1 pin 18/19
Ports on the Nano 328P: Serial0 pin Rx/Tx

What can be done about that? Thanks.

Ok what part is sending the byte back? The Nano? or is the Mega not detecting no connection?

That seems odd. You could have the Nano respond to the Mega with a different byte and if you get back what you sent, you know it's a phantom and the Nano is powered down.

Mega Code:

void RequestWatchdog(){
	if (!RFID.watchdog.requested){
		RFID.watchdog.requestTime = millis();
		RFID.watchdog.requested = true;
		if (RFID.backDoorClosed) {bitSet(serBuffer,3);}	// send the door status to the nano
		else {bitClear(serBuffer,3);}
		debug(F("Requesting rfidWatchdog..."),loglevel.toSerial);

void serialEvent1() 	//data from nano received
	if (serialNano.available() > 0) 
		RFID.watchdog.status = true;
		RFID.watchdog.requested = false;
		RFID.serialData =;
		if (RFID.serialData != serBuffer)		// this is not the rfidWatchdog
			RFID.readerError = bitRead(RFID.serialData,1);
			RFID.masterAlarm = bitRead(RFID.serialData,2);
			RFID.systemArmed = bitRead(RFID.serialData,3);
			RFID.validKey = bitRead(RFID.serialData,4);
			RFID.cardRead = bitRead(RFID.serialData, 5);
		if(RFID.validKey){debug(F("RFID key valid."),loglevel.normal);}
		if(RFID.cardRead && !(RFID.validKey)){debug(F("RFID: Invalid key"),loglevel.min);}
		if(RFID.masterAlarm){debug(F("RFID Master ALARM"),loglevel.min);}
		if(RFID.systemArmed){debug(F("RFID System Armed"),loglevel.min);}


void serialEvent()		//Watchdog request from mega 
	byte watchDog;
	if (Serial.available() > 0) {
		watchDog =;
		Serial.write(watchDog);		//bounce back
		backDoorClosed = bitRead(watchDog,3);	//Bit 3 holds the door info from the Mega

OK so now I have changed the response from the Nano to the Mega to:

		watchDog = 255; //;
		Serial.write(watchDog);		//bounce back

When the Nano is powered, I get 255. When it is not powered I get what I sent. I probably need to increment the number in the Nano to distinguish between phantom and real response.

Thanks guys!