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about the max LENGTH of SN65HVD12

Other Parts Discussed in Thread: SN65HVD12, SN75HVD12

Dear Sirs

 I have some issue about the max LENGTH of SN65HVD12

The following link is The RS-485 Design Guide.

From the equation: Lstub<= tr/10*v*c and datasheet of SN75HVD12

The max tr is 300ns.

So the max length <= 300ns/10*78%*c=22.932 ft ?

Is Maximum Cable Length 4000 Feet for Rs-485?

Big difference. Could you help me about this? Thanks


Bus Termination and Stub Length (from "The RS-485 Design Guide"):


datasheet of SN75HVD12:





  • Hello Nat

    You are right in both cases and yes is a big difference. The first part that you talking about, is the stub length, in next figure you can see two examples about it.

    The section where you see the formula is Bus termination and Stub length and in table 1 you can see different tr with different data rate and different stub length. On the other hand we have other section, Data Rate vs Bus length, and in this case is different connection (you don't use the same formula) you can see an example in next figure.

    Also you need to be careful, because this connection depend of the data rate, in figure 7 on the same file you can see a graphic where you can see the response of the cable length vs data rate.

    I hope this information can help you and please, let me know your comments.

    Best Regards


  • Hello Nat,

    Also to clarify, max stub length is calculated from the minimum differential rise time not the maximum. So in this case the maximum stub length is 7.67ft, which is different than bus length as Francisco pointed out. The bus length can be much longer up to 4000ft but this is a trade off with data rate and the number of nodes in the network. 

    Best Regards,


  • Dear Francisco and Casey
    Thank you for your great support.

    Could you provide the SN65HVD12 test report for ANSI standard TIA/EIA-485-A.

  • Dear Sirs
    Is any information about SN65HVD12 test report for ANSI standard TIA/EIA-485-A?

  • Hi Nat,

    Sorry for the delay. The electrical characteristics of the HVD12 were specified so that they either meet or exceed the requirements of the TIA/EIA-485 standard, so there is not a separate test report available. Instead, its compliance is ensured through the minimum and maximum specifications given already in the datasheet. If you have any other questions, though, please let me know.