TX RX Only working when connected to the pc

hey, i have a nodemcu and a Arduino board.
and the Arduino board serial printing a string and its catching by the nodemcu via TX and RX pins.
but my Arduino only sends that string when i connect it to PC.
when i power it from an external source, it's not sending the data!
why is that......

It's something to do with either your hardware, or your software. You posted in "hardware" so I guess you suspect that... without any details, only you can guess what is wrong.

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somehow when i connect nodemcu ground and Arduino ground together. it worked!

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Yes, you need a common ground. Because, electricity travels in circles!

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