How do you assign an address with RS485 - it is done with your firmware code. In order to achieve this you need a 'PROTOCOL' which incorporates some means of addressing. As to which protocol you use (there are many to choose from) or whether you invent your own - these are the decisions you need to make. Each protocol has its strengths and weeknesses - it all depends on your application. I normally use the Modbus protocol for my industrial instrumentation. See www.modbus.org where you can download the specification. This is what is known as a "master/slave" protocol - one device is the master that issues request, the other devices are slaves. The slaves can only respond when they get a valid request from the master for the assigned address. To set the slave address, one might have dip switches to set the device address. Again, there are many choices. In my equipment I have a lcd and setup menu, so the user can set the address and it is stored in eeprom.

If you can explain more about your requirements, we may be able to give you better solutions.