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SN65HVD3088E: RS485 impedance control.

KIYOSHI KOGURE
Prodigy 150 points
Part Number: SN65HVD3088E


Tool/software:

Dear

I am having trouble with RS485 impedance control.
Uses a multi-layer build-up board

However, due to the structure, it is not possible to control the impedance of 120 Ω differential on all outer and inner layers

I don't think this case is unique.
In this case, is it possible to design it with 100 Ω differential?

Any advice please?

  • 0
    TI__Genius 14632 points

    Kiyoshi-san,

    I have a couple questions for you:

    • Could you share your schematic and layout?
    • What exact type of cabling are you using? (particularly its characteristic impedance)
    • What termination resistor values are you using?

    It is possible to use 100 Ω characteristic impedance as this is actually the RS-422 standardization that is built into most RS-485 devices. Some of the electrical characteristics are tested at 100 Ω.

    However, 100 Ω is not recommended as it may cause more reflections and decrease signal integrity on the bus if it is not impedance matched. This article explains why improperly matched buses can cause issues: https://www.ti.com/document-viewer/lit/html/SSZTB23

     

    Let me know if you can share your schematic,

    Ethan