We are trying to connect the roborio over the digital output/input. The robot rio would output on the digital port to the arduino digital input. The question is the wiring. Would you have to run power and ground from the rio to the arduino? Or could you just run the signal wire from the rio to an arduino digital port?
If i understood you correctly, you need to connect ground and signal wires. The Arduino reads high if the signal level is high relative to ground. You don’t need the power wire unless your Arduino isn’t powered form another source.
If you want to make communication between the roborio that can send and or receive messages (more than high or low) you should connect them with I²c port or RS232(serial). It isn’t harder than digital writing and it adds a lot of functionality