The world of data communication is very complex. Data communication is concerned specifically with the issues that must be considered when communicating data between two devices (generally computers).
There are basically 3 modes of data communication:
Simplex: Data travels in one direction (from A to B). An example of a simplex link would be scoreboards such as those used in hockey, basketball, or other sports. The information is entered at a console by the score/timekeeper and sent serially to large displays that everybody can see.
Half-duplex: Data travels in one direction (from A to B) and then the other direction (from B to A) but not at the same time. The RS-485 is half-duplex.
Full-duplex: Data can travel in both directions at the same time.