I have an existing closed system that exposes several inputs that I would like to drive with an Arduino. The existing system (Neptune Apex Controller with break out box) is designed to sense the closing of dry contact switches between the input connector and the ground connector. The obvious easy solution would be to use relays to close the connection, however, I will need to change the input state every second or two which would burn a relay out in a matter of months if my math is correct.
Ideally I'd like to use an Arduino digital output pin to pull the existing system's input to ground. However, this means that both the existing system and my Arduino need to have the same ground voltage. I can't power the Arduino from the existing systems power supply easily since it's a black magic box with no details. All I know is that it uses 12V. I don't know if the supply is floating or not in relation to earth ground.
What would be the safest way to connect the Arduino to the existing system? Is it possible to use a floating 5V supply and simply connect the negative terminal from the 5V supply to the ground of the existing system? I'm not too familiar with what happens when the ground of floating and non floating or floating and floating power supplies gets connected. I would think that if the ground of the two supplies are at different potentials then I would have current flowing from one ground to the other which might burn out the existing systems circuitry.