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SN888C: SN888C RS485 Transceivers Technical support

Caliber
Intellectual 825 points
Part Number: SN888C

Tool/software:

Hi,

    We are using SN888C IC for RS485 Interface in our project.

    Controller is interfaced with PC using RS485 converter, rarely communication isn't working. When we swapped wire of A and B, Working fine on that time.

   After device power up on particular period, It's not working. Again, we swapped the A and B pin and its working properly. Same way, it's going on continuously.

   we thought the problem due to bus polarity automatic detection in this IC and can't find the root cause of the problem.

  Please find our device circuit design in the attachment. We are using the same design for both Rs485 Master and slave mode.

  Kindly refer and provide your guidance which is very helpful for us.

Thanks and Regards,

Dhanush.D,

  • 0
    TI__Genius 13352 points

    Hi Dhanush, 

    Based on your description, I agree that it seems that the bus polarity detection is the issue, and I believe it is not setup correctly.

    Are there failsafe resistors anywhere in the design (either the slave or master)? Failsafe biasing is required for the device to properly define the correct bus polarity orientation on start up. I have attached the Device Information section form the datasheet below (noted specifically resistors R_FS):

    This document further details failsafe biasing with equations to calculate the resistors needed for your system: https://www.ti.com/document-viewer/lit/html/SSZTAK0

    If your system already has failsafe biasing, please let me know and we can work on a new solution.

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Intellectual 825 points

    Hi Ethan,

    Following your suggestion, we tried to adjust the R_FS(645 ohme) so that the A line was pulled up and B line was pulled down, but still the issue persisted. 

    Thanks and Regards,

    Dhanush.D,

  • 0 in reply to Caliber
    TI__Genius 13352 points

    Hi Dhanush,

    Could you show an image or a schematic of the new setup with the resistors you implemented?

    Additionally, could you share an image of the top of the device so I could check it in our internal system? 

    Best regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Intellectual 825 points

    Hi Ethan,

    For your reference, I've included the schematic with the resistors implementation....

    Kindly refer and provide your guidance which is very helpful for us.

    Thanks and Regards,

    Dhanush.D

  • 0 in reply to Caliber
    TI__Genius 13352 points

    Hi Dhanush,

    Could you please upload the schematic again? It did not upload correctly. I also recommend a Powerpoint or Excel document with the images inside to ensure they upload correctly.

    -Ethan 

  • 0 in reply to Ethan Sempsrott
    Intellectual 825 points

    Hi Ethan,

         Herewith i have attached the schematic file in the excel document for your reference.

    5153.Book1.xlsx

    Thanks and Regards,

    Dhanush.D

  • 0 in reply to Caliber
    TI__Genius 13352 points

    Thank you, I can see the updated schematic now. 

    If the fail-safe resistors are added properly, then the bus will have an established state when idle. This lets SN888C detect the bus orientation within time t_FS (about 80ms). After this the device will need to be powered off then on again for it to detect the polarity. 

    Did you add fail-safe resistors to the other device? To clarify, these should only be added to one device.

    What is the other device on the bus?

    Could you show waveforms both when the device is communicating and when it is not? 

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Intellectual 825 points

    Hi Ethan,

    I should only add resistors to the master device.

    For your reference, I have included the waveform images when the device is communicating as well as when it is not.

    Thanks and Regards,

    Dhanush.D

  • 0 in reply to Caliber
    TI__Genius 13352 points

    Thank you for the info Dhanush.

    Based on the information you have provided, the device should be operational. Because the SN888C is an older device, this could likely be a quality issue. At this point I doubt your setup is the issue. 

    My recommendation would be to visit our Failure Analysis page which will walk you through the return process. 

    Best regards,

    Ethan