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Original question:

SUPPLY VOLTAGE OF SN65176

SN65176B: A unormal byte 0 When start receive Data for SN65176BDR Issue

user5158211
Prodigy 130 points

Part Number: SN65176B

Issue :

SN65176BDR Under RS-485 protocol ,When start receive Data , A unormal  byte 0  occured in the fist data every time. , Then  the  data error.   But when we changed the device to  Maxim  MAX485ESA+T , it get OK, No the issue  1byte 0 in the fist data.   later  we changed to a  TI SN65LBC176DR, it still  get OK too.

Question:

1:Any special difference between  SN65176BDR  or    Maxim  MAX485ESA+T (low cost）?

2:How to improve SN65176BDR design to solve the Issue.

Thank you!

  • 0
    TI__Mastermind 25705 points
    Hi,
    Sorry to hear the issue. There is not much difference on the function between SN65176BDR and SN65LBC176DR. One thing stands out is SN65176BDR consumes much more current than SN65LBC176DR. I'm not sure if it's the impact. What's the data rate in your application? Do you have any termination resistors on the bus? When there is an error at the byte 0, do you mean it's the start bit, or the first bit in the data? What's the bus state before that? It would be nice if you provided some hardware debug information, such as waveform.
    Regards,
    Hao
  • 0 in reply to Hao L
    Prodigy 130 points

    Hi Hao:

         Thank you for your reply. The further information is ：1) Bude Rate 57600.   2)  The 120 ohm Resistor    removed or keep  ,no  any improve on the issue. 3) The unnormal 0 byte occured when the RS485 from output change to Input . And only the SN65176B isue , no SN65LBC176DR and Max485esa+t issue.

         Any suggestion  freely  relply ASAP

         Thanks!

  • 0 in reply to user5158211
    TI__Mastermind 25705 points
    Hi,
    Thanks for your information.
    After the RS485 changes from output to input, who is driving the bus? If the bus is idle, with a 120Ohm resistor, the voltage on each bus would become equal. What byte do you expect to receive?
    If there are more than one nodes on the bus, if it's SN65176B changes from output to input, what's the receiver state during the transition?
    What byte other nodes (if they're not SN65176B) receive? Do they see the same error?
    You can try to add fail-safe resistors on the bus, like 500Ohm pull up on A side, 500Ohm pull down on B side, with and without 120Ohm termination resistor.
    Please let me know how it works.
    Regards,
    Hao
  • 0 in reply to Hao L
    TI__Guru 62920 points
    Hello,

    Have you had a chance to look into Hao's questions/suggestions above? Or, have you had any progress in resolving this issue?

    One other thing that might be helpful is to probe the A and B signal lines on an oscilloscope. That way we can see what is happening on the bus that corresponds to the incorrect data you are detecting.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 130 points

    Hello,No special issue on oscilloscope  and no  improve  to add  pull and push resistor ,we have tested  others  TI RS485 device and   OK,   well used now ,Thank you!