Energizing the entire board with a transducer signal

I'm building a project that uses 5 Arduino uno boards. 4 of them control and monitor 4 electrolysis devices, while the 5th board controls and monitors some of the infrastructure such as pressure transducers and fans. The pressure transducers are 2 wire, 4-20 ma devices powered by 24v DC. One such transducer is distributed to all 5 UNOs. First by converting the 4-20 ma signal to a 1 to 5 v through a 250 ohm resistor, then the 1-5 v signal fans to (distributed to) all 5 boards, 4 are brought to A3, and one to A1 (See attached print).

This project is powered by 120vAC. The neutrals and grounds are separated from each, but each brought to its own common point. There are 4, 15 Volt power supplies that power the 4 electrolysis cells and 1, 24v supply powers various transducers and fans. The UNO boards are powered buy wall warts (AC to DC plug-in modules).

Now here is the part that I do not understand. While testing the shared transducer signal on one board the other boards were not plugged in. When I applied a test signal to one UNO all 5 UNOs powered up!

If anyone can shed some light on this I would appreciate the info. I don't have a real schematic of the UNO but it must be that the presence of a voltage on an analog input is somehow able to energize the VCC on the entire board. Could that be right? Wouldn't that mean significant "cross talk" or some other kind of interference or confusion of signals? Best regards Gene

Yes, this is called “parasitic powering” because you have pins from one processor connected to pins from another.

The “HIGH” outputs fed from one are causing the protective diodes to conduct in the otherwise un-powered one and feed power from the port pin back to its Vcc You are asking for damage. You have to make sure pins from one Arduino are not connected directly to another for any reason and in fact, that nothing can apply voltage to any port pin of an un-powered Arduino.

attachment for above

HECS Main Power - 040915 v1.pdf (136 KB)

Thanks Paul

  1. So I can’t connect an analog signal to 5 different cards? What if they are already powered up?

  2. How about a digital out from one card to digital ins on the 4 others (using the analog pins as digital input)? Will I get parasitic-powering" if the input cards are unpowered?

  3. Just to be sure I get this right, by connecting 4 different board’s analog inputs together (in parallel) I can power the unpowered boards by driving a current backwards through a diode on the boards that are not powered up normally.

  4. Finally, I can not send signals to an unpowered board period Right?

Thanks for the “parasitic-powering” info!

Use a 10k resistor between the 5volt signal and an Arduino input pin.
So one resistor per Arduino, close to the Arduino.
That won’t affect readings, and reduces parasitic powering to <500uA.

Don’t use (non-lineair) diodes.

Note that when one or more Arduino’s are off, the readings of the other ones are slightly reduced.