Power supply through serial pins?

Hi all,
today I observed an unexpected behviour:

I have two arduino nano (let'a call A and B), connected according to this schema:

A GND -------> B GND
A TX -------> B RX

Arduino B does not have (till now) any power supply.

As I plugged the USB cable in arduino A, I saw that the led "ON" in arduino B lighted!!!
So some power flows through serial pins!!!

On a single arduino I measured the resistence between pin 5V and RX: there is some low resistence!
So the pins are in someway connected!!!

I'm wondering if this is ok.
I do not want to burn arduino A because it can't feed arduino B.
Thanks for advices!

  1. Yes, it's normal. The default state of the transmit pin is HIGH.
  2. As you noticed, you can indirectly (and incorrectly) apply power via the IO pins. You should always have main power applied before the pins are connected (or at least at the same time).