Forwarding serial output from a Nano to Uno that's used as the ISP

Question

I'm trying to program a Nano by using an Uno as the ISP. So far, I've followed along with the example project for getting the Uno running as the ISP on https://www.arduino.cc/en/Tutorial/ArduinoISP.

After a bit of tinkering, it works, but the serial monitor won't work correctly. I'm uploading a super simple program that delays for 500 ms and prints to the log.

void setup() {
  Serial.begin(9600);
}

void loop() {
  Serial.println("test");
  delay(500);
}

I've wired the TX pin on the Nano to the RX pin on the uno. The monitor updates every 500 ms as expected after doing so, but displays ��� instead of the word test. When I tried wiring TX -> TX, the serial monitor does not update. Does anyone have any insight what's missing here?

Answer 1

As Chris Stratton suggests in his comment above:

You can't really connect an ordinary Arduino's serial pins to both another device and the on-board USB serial converter at the same time.

To solve your serial monitor problem use a separate USB serial converter and connect that to your Arduino Nano.

If you don't have access to a dedicated USB serial converter you could use software serial on the Arduino Nano. This allows you to connect the Arduino Uno Rx pin to the software serial Tx pin. You will also need to program the Arduino Uno with a program that re-transmits the data it receives back out to the PC serial monitor.