MAX485 Question

Now I want to use MAX485 for 2 Arduino communication with long distance. But after I tried, I noticed that used separated power supply. It cannot communicate. Is MAX485 can use with 2 separated VCC line but shared the same ground.

How about some more detail? Distance what power supply. Data rates… details please

I use 5V power supply both but from different source, One is from the USB port the other is from 12V Battery and step down to 5V. MAX485 use 28500 baud rate and the distance between two MAX485 is around 10-12 metre

Can you communicate over a short wire using a single power supply?

Can you communicate over a long wire (just make a big loop) and a single power supply?

What kind of wire? Can you post a picture?

Based on my experience with rs422 which is similar to rs485, i think you should be able to communicate using differential pair. You can find details here:

http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/•-Polarities-for-Differential-Pair-Signals-(RS-422.aspx

You should be able to communicate perfectly with separate psu's without common ground by connecting each transeivers A with A and B with B and add 120ohm terminating resistors betwewn A and B to avoid reflections.

Do i make sense ?

So sis you figure it out ?