Connecting Arduino#1 out pins to Arduino#2 in pins powers up Arduino#2 !

Other Hardware General Electronics
1

...without Arduino#2 being plugged in to USB or power socket.
Not properly powered up but enough for it to start running its sketch.

Both Unos.
The grounds are commoned.

Do I have to put a diode in line with the digital lines?

Any explanation and fixes gladly appreciated.

Thanks.

2

Any explanation and fixes gladly appreciated.

The "un powered" arduino is being back fed by the powered arduino via the connected ground and pins.

3

Ardunaut:
Hi

Its either on or off, nothing in between.

Sorry, I should have said intermittent with dim on board LEDs.

I'm controlling five Arduinos from ones output signals - the power thing is an unwanted artifact!

4

That is sometimes called "ghost" or "phantom" powering and it can damage both the input and the output Arduino.

You should not connect live signal leads to the inputs of an unpowered Arduino, but if you can't avoid it, a 10K ohm resistor in series with the input lead will usually prevent damage.

5

That is sometimes called "ghost" or "phantom" powering

or sometimes parasitic powering. It is not good, do not do it.