In a **Full-Duplex** system, communication can occur in both directions simultaneously. Old-style telephone service lines are a prime example of an FDX communication system; both parties are able to speak and be heard by the other party simultaneously, there is a constant channel each way between them.
In a **Half-Duplex** (also known as a **semiduplex**) communication can only occur one way at a time. If device A would like to send data to device B, but device B is already sending data to device A, then device A will have to wait until device B has stopped transmitting to send data.
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When it comes to RS-485, the total line count defines whether the system is half or full duplex. In a half duplex you only need 2 lines (DATA + & DATA NEGATIVE fff duplex RS-485 line you should have 3 lines (DATA TX, DATA RX, GROUND)