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SN65HVD82: Can AB line add pull-up and pull-down resistor?

Part Number: SN65HVD82


        RS485 chip ground all connected together, AB line is not pulled up and down (R66 R67 attachment schematic not added), communication is OK. Now that the aging machine has increased the quality of the

communication, it is suspected that the pull-up resistors are not connected. Therefore, the addition of a 4.7K pull-up and pull-down resistor indicates that the communication has improved.

        If I want to add 100 machines, if the pull-down resistor 47K, because it is a common ground, is not the pull-down equivalent resistance is 470 ohms? If the machine increases again, will there be a load pull death?

  • Hello,

    Thank you for your questions.  

    Can you clarify what the 4.7Kohm resistors are being used for in your network?  Pull-up and pull-down resistors can be used on an RS-485 bus to accomplish fail-safe biasing, but this needs only to be done once at one node.  You shouldn't need to place pull-up and pull-down resistors at each node.  

    An RS-485 transceiver is required to handle a common-mode load of 375ohms.  By placing pull-up and pull-down resistors at each node, you will increase the common-mode loading on the system significantly.  For 100 nodes, this will almost certainly disrupt communication.

    On a second note, the SN65HVD82 device contains integrated fail-safe biasing.  If the SN65HVD82 is used at each node in the network, then these pull-up and pull-down resistors are redundant for fail-safe biasing.  Could you clarify what you mean when you say that these resistors improve the communication?  Do you have a before-and-after comparison that you can share?

    Here is some information on fail-safe biasing, including some basic design calculations and trade-offs.  Please let me know if you intend these resistors for another use.  And please let me know if I need to clarify anything.

    Best Regards,

    Max Megee

  • Hello Max Megee,
    Thank you for you answer.
    The 4.7Kohm resistors is the R66 and R67 connected to the A B pin.