If you want to test RS485, then really you should use THREE or more devices.
Since RS485 is a multi-drop bus, you'll want to ensure that the data can be transmitted from any one node to every other node successfully.
One node will becomde the bus driving force by asserting the DE signal on the Max485 and de-asserting it on all the other nodes. You can assert the !RE (Receive enable) signal on ALL the nodes, even the transmitting node at any time, even when transmitting. This allows the transmitting node to "read back" it's own transmissions. This is useful for detecting collissions on the bus.
For example if you send 0xff,0xff,0xff but read back something other than that, you can deduce that there is eithew a) A fault on the bus, or b)some other node transmitted at the same time.
The bus should be wired point to point, from one node to the next, but not as a "star". Appropriate terminating resistors should be used on the nodes at the far extents of the bus, but not on the intermediate nodes.
I should add... Use the appropriate size and guage of wire, connecting all the A's together and all the B's together. A common ground is not necessary, since RS485 is a differential signal, however using a common ground can reduce current consumption.
