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SN65LBC184: SN65LBC184 Waveform issue

Yang Wu
Expert 3420 points
Part Number: SN65LBC184
Other Parts Discussed in Thread: SN65HVD72,

Hi Team,

When my customer uses SN65LBC184 and SN65HVD72 for the same circuit as below,

they find waveform differences on bus input/output A & B when the RS-485 starts working.

SN65HVD72 normal waveforms:

SN65LBC184 abnormal waveforms (overshoot):

Have you meet this issue before? Any ideas about this issue?

Thanks.

Best regards,

Yang

  • +1
    TI__Guru 53575 points

    Hi Yang,

    This looks like a bit of asymmetry in the signal lines from the SN65LBC184 set during initial powerup. Both lines seem to charge up initially, likely with on driver active and the other high impedance. The second line (blue) follows the driven line through the termination resistance in the network. I suspect the newer design of SN65HVD72 has a more symmetrical startup sequence that provides a cleaner initial transition to the system. 

    • Does the SN65LBC184 setup show similar behavior during signal transitions as well? Or does thi only appear during startup?
    • Is this currently causing a problem in the customer's system? Or are they only looking for an explanation for the behavior?

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 3420 points

    Hi Eric,

    It's only appear during Startup. Customer is only  looking for an explanation for the behavior. Could you provide some detailed explanation if it's possible? Thanks.

    Best regards,

    Yang

  • 0 in reply to Yang Wu
    TI__Guru 52698 points

    Hi Yang, 

    The SN65LBC184 is being operated within absolute maximum VCC range but is outside the range for recommend operating conditions. For the LBC, VCC should be 4.75V < VCC < 5.25V. 

    SN65HVD72 is being operated correctly. Datasheet states 3V < VCC < 3.6V. 

    I am curious to see if powering the LBC within the recommended operating conditions will have any affect.

    For clarification, is the value of the bypass capacitor 10 x 10^4 pF = 100,000pF = 0.1uF?

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Expert 3420 points

    Hi Tyler,

    For LBC184, VCC = 5V. For HVD72, VCC = 3.3V. All most no bypass cap. thanks.

    Best regards,

    Yang

  • 0 in reply to Yang Wu
    TI__Guru 74322 points

    Yang,

    Thank you for the information, we will get back to you by the end of the day on 11/30/2021 CST. 

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Guru 74322 points

    Yang,

    Sorry about the delay here, we are still looking into this. Thank your for your patience.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Guru 53575 points

    Hi Yang,

    It appears that this behavior is caused by the shift in the bus common mode which occurs during the handoff in a mixed-voltage bus. What occurs here is that the common mode shift from 5V / 2 to 3.3V / 2 means there's a common mode current on the bus. This sudden current through the inductance of the cable produces a small voltage rise (or fall) in the common-mode of the bus. Once the common-mode voltage equalizes at its new value, this current stops and the voltage settles. This will not occur again until there is another handoff where the common-mode shifts again. Because this noise does not appear in the differential signal, it does not pose a concern for the integrity of the data during this time. 

    Let me know if this is clear and if there's anything I can elaborate on. 

    Regards,
    Eric Schott