If I have two Arduinos, and I want one pin on Arduino A (output) to be a signal to Arduino B (Input). Can I configure Arduino A's output pin as output, and configure Arduino B's input pin as INPUT_PULLUP? That way, Arduino B will digitalRead a TRUE if ArduinoA Output HIGH, and digitalRead FALSE if ArduinoA Output is low? And if the output is not connected, Arduino B will be sure to read a HIGH signal because of the pullup instead of float?
The thing I'm not sure about is that Arduino A's Vdd could be slightly different than Arduino B's Vdd. Is that OK?
On the flip side, when the Arduino A's output is set to low, will there be current flowing, basically from B's Vdd, across the 20k internal pullup resistor, to ground on Arduino A?
The two Arduino have their ground pin tied together.