user47065: Can I connect the digital out of one arduino to a digital input of another? To detect a high/low state...
One powered in one room on USB. The other in another room separate psu (connecting the grounds).
I know I can use tx/rx serial. But if the other way would work, less programming and 1 wire less.. 

user2881: Yes, you can. As you noted, you do need to connect the grounds of both Arduinos. 
For some extra protection you could put a resistor between the output and the input. For example a 1kOhm. That way, if you accidentally set both pins as outputs, or plug the wire in the incorrect pin, you won’t get a dead-short. 

47065: <p>Can I connect the digital out of one arduino to a digital input of another? To detect a high/low state...
One powered in one room on USB. The other in another room separate psu (connecting the grounds).
I know I can use tx/rx serial. But if the other way would work, less programming and 1 wire less.. </p>
 2881: <p>Yes, you can. As you noted, you do need to connect the grounds of both Arduinos. </p>

<p>For some extra protection you could put a resistor between the output and the input. For example a 1kOhm. That way, if you accidentally set both pins as outputs, or plug the wire in the incorrect pin, you won’t get a dead-short. </p>
