Input/Output read/write without using Ground or Power/Source possible?

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hi everybody!

i have basic (and perhaps stupid) question connecting 2 digital pins (output , input) without using ground or 5v/3.3v output.

so, simplest example can think of, digital pin 9 connected push button connected resistor(10kohm) connected pin 8.

an ascii drawing this:
d9 - pushbutton - resistor(10kohm) - d8

d8 digital input, d9 digital output.

in tutorials , examples, guys using ground or 5v/3.3v output.
i have read in documentation, that, if pin declared output, provides ~5v.
my assumption is, if declare pin input, grounded.
right?

and therefore circuit diagram should correct. :)
but correct or have connect pin power source or ground additionally (and if so, why?)?

thx!

yes! can work.  (you don't need resistor.)

the input & output pins both share common ground (which reference point).   so, if connect them together, both have same voltage, referenced same ground.   

now... if had 2 different arduinos , each powered separate batteries no common ground between them, connecting output 1 arduino input of arduino not work. 

a voltage needs reference or "potential difference", or voltage needs "across something" current flow.   with no other connection, "input" ardino wouldn't "see" 5v "difference" between input pin , ground.


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