Arduino - Serial communication - Does not work without computer

I’m new to the forum, and I am seeking help on an issue I found with Arduinos.

I have designed several standalone programs with no use of Serial link and these are all working fine without computer connected. Recently I have written a program that uses Serial for communication between 2 arduino UNO ( via RS 485 but it’s not the point ).

I have noted is that when Arduinos are connected to a computer with USB ( and also to each others via RX/TX cross-wired ) it does work ( RX and TX leds are flashing accordingly ), but when no computers are connected ( and whatever power supply I’m using, +9 on the appropriate, +5 on Vin or USB plug ) nothing happens anylonger, and the RX/TX leds are not flashing anymore.

Has anyone already faced this situation ?
Looks like a kernel bug to me ?

All right
I've checked with an oscilloscope, and the leds are not flashing when you are not connected to a computer, however the serial information is sent.

So, it's no bug...

The RX/TX lights are driven by the USB serial adapter, and reflect what it sees - on official boards and faithful clones (ie, ones with a 16u2 for serial connectivity).

On unfaithful clones (ie, CP2102/CH340G based ones), they're normally driven straight off the TX/RX lines with a suitably large resistor in series. Frankly, this is frequently more desirable :wink: