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THVD1420: some times data getting garbage

sankar itraju
Prodigy 50 points
Part Number: THVD1420

Tool/software:

HI TI team,

We have small issue with THVD1420 RS485 communications some times am getting garbage value on RS485 bus.could you please suggest me where is the mistake ?

and here am attaching my circuit and PCB desigh Please go through it Thank you.

This is my circuit

This is my PCB desigh file

  • 0
    Guru 316690 points

    The schematic and the PCB do not match.

    What is the purpose of D16? RS-485 signals are bidirectional.

  • 0 in reply to Clemens Ladisch
    Prodigy 50 points

    HI Clemens Ladisch 

    Thank you for your quick response.

    Apologies for the confusion in my earlier communication — the schematic I had attached contained an incorrect part name. Please find the correct schematic attached with this message.

    The same design has been used successfully in my previous implementation, where it worked reliably with a setup consisting of one master and one slave.

    However, in the current application, I am using a single master and over 120 slave devices. Under this setup, I occasionally receive incorrect or invalid data.

    Kindly review the attached corrected schematic. I would also like to confirm the following:

    • In the earlier schematic, you mentioned the use of D16. In this updated version, can I safely remove D14

  • 0 in reply to sankar itraju
    Guru 316690 points

    You must remove D14. You also must remove R36 on all nodes, except at the two ends of the bus. You also should remove R35 and R39 on most nodes.

  • 0 in reply to sankar itraju
    TI__Guru 52172 points

    Hi Sankar,

    Clemens is correct - D14 shouldn't be there - it will cause garbage data and only the 2 end nodes (start and end node) should be terminated with a 120 ohm resistor. 

    Ideally the network topology is daisy chained to minimize unterminated stub lengths - for this device your unterminated stub length max (where length is measured as deviation from main trunk of bus) should be <= 350mm - this assumes that phase velocity of the transmission media you are using is ~78% of c - which is pretty typical in RS-485 systems.  Max unterminated bus length : L <= t_tr(differential-minimum time listed)/10 * Vp - where t_tr for this device has a smallest value of 15ns listed as its differential transition time and Vp is assumed to be 78% * c - so there could be some difference depending on the phase velocity of the medium you are using. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 50 points

    Thank you so much for your quick replay Clemens and Parker Dodson

    actually am using daisy chained mechanism am not using termination resistors for slaves except start device and end device

    I will remove the components what you suggest me then get back to you is there any changes in PCB  design? Thank you

  • 0 in reply to sankar itraju
    TI__Guru 52172 points

    Hi Sankar,

    Please let me know if the removal of the components help. Also thank you for the confirmation that you are using a proper network setup. 

    As for the PCB - it looks okay. Ideally you want the A/B output lines differentially coupled - but the A/B traces are really short on the PCB and this isn't that fast of a device so it shouldn't be causing that much issues - if you do a revision however I would at least try to make the traces the same width and as close as possible to the same length - as that is just best practice - but I really doubt that is causing the issues I do think the main issue originates from D14. when "B" goes high you will have trouble sending that signal due to the diode. The 712 diode you have should be enough w.r.t. protection needs. 

    Best,

    Parker Dodson

  • +1 in reply to Parker Dodson
    Prodigy 50 points

    Thank you Parker Dodson for your suggestions Please let me check my hardware with your guidance then I will get back to you Thank you very much.