user49175: I have multiple arduinos that are each powered by a 12v battery through a 5v regulator.  All arduinos share a common ground.  Can I simply connect the digital output from one arduino into the digital input of another?  I'm worried the (small) voltage differences between each regulator could cause a problem.

user31974: It's ok if the distance between the two devices are not too big. (couple meters)
Since the valid logic level (voltage) ranges are pretty wide a small difference won't affect the operation of the devices. Arduino uses TTL (5V) logic so let's see the level diagrams:

If you see the output level ranges (on the right) are more strict than on the input side (on left) so the logic value remains valid even if a small voltage drop or noise occurs on the wire.
Make sure you won't connect two outputs directly since it can damage the microcontrollers. Connecting two inputs are totally safe.
EDIT: If you want to cover longer distances you need to use a balanced line  e.g. differential I/O-s, RS-485 or CAN. These are not default Arduino features so you'll need additional electronic to use them.

49175: <p>I have multiple arduinos that are each powered by a 12v battery through a 5v regulator.  All arduinos share a common ground.  Can I simply connect the digital output from one arduino into the digital input of another?  I'm worried the (small) voltage differences between each regulator could cause a problem.</p>
 31974: <p>It's ok if the distance between the two devices are not too big. (couple meters)</p>

<p>Since the valid logic level (voltage) ranges are pretty wide a small difference won't affect the operation of the devices. Arduino uses TTL (5V) logic so let's see the level diagrams:</p>

<p><img src="https://sub.allaboutcircuits.com/images/04312.png" alt="TTL levels"></p>

<p>If you see the output level ranges (on the right) are more strict than on the input side (on left) so the logic value remains valid even if a small voltage drop or noise occurs on the wire.</p>

<p>Make sure you won't connect two outputs directly since it can damage the microcontrollers. Connecting two inputs are totally safe.</p>

<p>EDIT: If you want to cover longer distances you need to use a <a href="https://en.wikipedia.org/wiki/Balanced_line" rel="nofollow noreferrer">balanced line</a>  e.g. differential I/O-s, RS-485 or CAN. These are not default Arduino features so you'll need additional electronic to use them.</p>
