RS-485 is one of multi-drop communication that allows us to ‘talk’ to multiple devices at the same time.
According to the standard, up to 32 devices can be connected at the same time. Maximum distance from end to end can be up to 1200 meters.
By using repeaters, however, both the total number of devices and maximum distance can be extended.
RS-485 network can be used as a two-wire or four-wire network.
The four wire network would be bidirectional (a simultaneous two way conversation can happen) whereas the two wire network works only in one direction.
It is either a 3 or 5 wire system. The third or fifth wire is actually a ground wire.
The RS-485 disadvantage is that it is harder to program, because it uses the same 2 wires to send and receive data. And in any given network, only one node can transmit data, other nodes can only receive at that particular moment. On the advantages side, it supports long distance communications with no problems. It also uses lower interface signal levels than the RS-232, which makes the interface circuit harder to damage.