Arduino master/slave communication using RS485
First of all, your connection is just wrong, you should make the RS-485 bus as a straight line, with a 120 ohm resistor at the beginning and another one at the end. Each slave should tap into that line as close as possible to that line, look for datasheets and you will understand what I am talking about.

Second, you can use Modbus as a pretty good and easy to use communication protocol.