Nano sends via serial without power connected

Hardware Nano Family Classic Nano
#1

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.

#2

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

#3

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.

#4

Mega Code:

void RequestWatchdog(){
	if (!RFID.watchdog.requested){
		RFID.watchdog.requestTime = millis();
		RFID.watchdog.requested = true;
		serialNano.flush();
		if (RFID.backDoorClosed) {bitSet(serBuffer,3);}	// send the door status to the nano
		else {bitClear(serBuffer,3);}
		serialNano.write(serBuffer);		
		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 = serialNano.read();
		delay(5);
		debug(String(RFID.serialData),loglevel.toSerial);
		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);}
	}
}

Nano-Code:

void serialEvent()		//Watchdog request from mega 
{	
	byte watchDog;
	if (Serial.available() > 0) {
		watchDog = Serial.read();
		Serial.write(watchDog);		//bounce back
		backDoorClosed = bitRead(watchDog,3);	//Bit 3 holds the door info from the Mega
	  }
}
#5

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

		watchDog = 255; //Serial.read();
		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!