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[FAQ] How many nodes can you connect to an RS-485 bus?

Anthony Viviano
Prodigy 460 points

How many nodes can you connect to an RS-485 bus?

  • 0
    TI__Prodigy 460 points

    To estimate the maximum number of bus loads possible, RS-485 specifies a hypothetical term of a unit load (UL), which represents a load impedance of approximately 12kΩ. The Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA) RS-485 standard mandates a maximum of 32 ULs that can be added  to an RS-485 bus. The UL of a node is calculated by the worst-case ratio of the input voltage divided by the leakage current, as shown in equation 1:

              

    Once you’ve established the UL of the node, equation 2 calculates the maximum number of nodes:

     

    Most of TI’s isolated RS-485 transceivers have a UL of 1/8, translating to a maximum of 256 nodes on an RS-485 bus.

    For more information about UL and the number of nodes that can connect to an RS-485 bus, see the Analog Applications Journal article, “The RS-485 unit load and maximum number of bus connections.”