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SN65HVD3082E: [FAQ] External pull-up & pull-down resistors for SN65HVD3082E

LAW HUNG
Prodigy 10 points
Part Number: SN65HVD3082E

Hi Experts,

May I know for the followings about SN65HVD3082E:

1. Can we add external pull-up and pull-down resistors for SN65HVD3082E?

2. What would be the effects of "adding" and "not adding" pull-up and pull-down resistors? We are communicating with some external devices that may not being using SN65HVD3082E.

3. Will it be OK if we use 10kOhm pull-up and pull-down resistors? We expect to communicate with at least 16 standard unit loads.

Thanks.

  • 0
    TI__Guru 50795 points

    Hi Hung,

    Which nets are you referring to? If you are asking about biasing resistors on the digital signal lines (R, D, RE, DE) then yes, this is fine. These pins have integrated biasing resistors, so exclusion of these outside of the device is also fine. 

    For biasing resistors on the bus, this sounds like a failsafe biasing network. This is a common practice for placing a floating bus into a known state, but there are considerations that need to be made to ensure this is done correctly. Check out this application report for more information. 

    Let me know if you have any questions on this material of if I misunderstood your question. If not, please let us know by clicking the resolved button. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 10 points

    Hi Eric,

    Thanks for your reply. I am referring to the AB bus. As according to the datasheet, no external biasing is required for SN65HVD3082E.  I just wonder if there will be any pros and cons for adding additional external failsafe biasing network for SN65HVD3082E.

  • 0 in reply to LAW HUNG
    TI__Guru 50795 points

    Hi Hung,

    The SN65HVD3082E has internal failsafe biasing on the receiver, so the receiver will behave predictably for a floating bus without the need of an external failsafe biasing network. An external network may still be used to provide biasing for other nodes on the bus. External networks also increase the noise immunity of the system while in a floating state. Other advantages are noted in the linked note. Let me know if you have questions on any them. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 10 points

    Hi Eric,

    "Internal failsafe bias on the receiver" means on the output R, not on the bus?

    Since I am communicating with some externally connected devices (like power meters), that means in case those external devices are using legacy transceivers, then I need to add external biasing network?

    Is there any limitation on how many biasing networks can be used within the same bus? I asked this because we have no control over whether the devices we are communicating with already have biasing networks or not.

    Thanks,

    Hung

  • +1 in reply to LAW HUNG
    TI__Guru 50795 points

    Hi Hung, 

    LAW HUNG said:
    "Internal failsafe bias on the receiver" means on the output R, not on the bus?

    No. The internal biasing is on the RS-485 receiver (pins A and B). This is shown as a current source in the block diagram for these inputs. 

    LAW HUNG said:
    then I need to add external biasing network?

    External biasing is only necessary when other nodes on the network are not biased internally. Because legacy transceivers may be used in this case, it is likely a good idea to include external fail-safe biasing somewhere in the network. However, it is likely that if fail-safe biasing was required for these legacy devices, that those nodes already include such a circuit. If so, you should not include a second biasing network to avoid overloading the bus. 

    LAW HUNG said:
    Is there any limitation on how many biasing networks can be used within the same bus?

    Yes. The biasing network adds load to the bus. If many of these circuits are added, or one circuit that is too strong is added, it could make communication difficult or impossible for the transceivers. This is outlined in detail in the linked document. Please review the calculations included there. 

    If you are not in control of biasing elsewhere in the network, it may be best to only worry about the local node. In this case because SN65HVD3082E does not require this network, I would recommending against including one in your design. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 10 points

    Hi Eric,

    Thanks for your help. We got better understanding on the issues now.

    Regards,

    Hung