A duplex communication system refers to any point-to-point (PTP) system composed of two or more connected devices which can communicate with each other in both directions. There are two common duplex communication systems: Full-Duplex (FDX) and Half-Duplex (HDX).
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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.
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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 capable of half or full duplex. In a half duplex you only need 2 lines or 1 channel (DATA+ & DATA-) duplex RS-485 line you should have 4 lines or 2 channels (DATA TX+, DATA TX-, DATA RX+, DATA TX-).
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For further information on what half and duplex refers to, please see [[Communications/Serial]]
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### Common Ground
One big issue that I have often seen in access control system installations which utilise the RS-485 standard for communication is no common ground wire being carried the full length of the data line. There's a lot of contradicting information online as to whether or not a common ground is required. To put it simply, you can happily run RS-485 data lines with no common ground over small distances for two reasons: