RS-485 has the ability to multi-drop, and RS-422 does not. RS-422 will have a RS-422 driver chip that does NOT go into a tri-state mode. This means that the transmitter and receiver are ON all the time. Since the transmitter does not go to a OFF state, or Tri-state, mode, then multiple RS-422 devices cannot be connected in a Multi-drop mode, or the Transmitters of the deviceswould be in contention with each other. RS-485 devices have the ability to turn the transmitters off, or Tri-State, when they are not transmitting. This allows multiple devices to be daisy chained together in a Multi-drop mode, and as long as only 1 device is talking at a time, and the others are OFF, there will be no contention on the bus.