For further information on what half and duplex refers to, please see [[Communications/Serial]]
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### Common Ground
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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: