• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 32 subscribers
  • Views 562 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN65HVD485E: More Specific Conservative Cable Lengths (than are reasonably estimable graphically from the provided figure)

Alex Suchko
Prodigy 30 points
Part Number: SN65HVD485E

Hello TI,

I have an RS-485 industrial control application with the SN65HVD485 that could benefit from higher bit rates (bps), but of course at the same time max cable length is an important specification for the application.

The SN65HVD485E datasheet graciously provided by TI provides a figure "Figure 19. Cable Length vs Data Rate Characteristic" which has a grey region specifying the Conservative Characteristics max cable length versus bit rate.

I must admit I'm a bit rusty with reading log-log plots so without the extra gridlines it's currently difficult for me to graphically estimate specific values.

Could you please tell me what this Conservative Characteristic region's max cable length in units of feet evaluates to at the following bit rates:

  9600 bps

115200 bps

921600 bps

Thank you,

Alex

  • 0
    TI__Guru 62910 points
    Hi Alex,

    Sure, I can help give a little bit of detail here. At 9600 bps, you are low enough in frequency that the maximum distance is limited more by the DC resistance of the cabling than any higher-order frequency-dependent attenuation effects. This limit corresponds to the horizontal boundary of the "conservative characteristic," which is at 4000 ft. At 115.2 kbps you are just starting to see frequency-dependent effects; the max distance would be about 3500 ft. there. At 921.6 kbps the limit would be between 400 ft. and 500 ft. due to higher-frequency attenuation in the cable.

    Keep in mind that these are guidelines from the RS-485 standard based on typical transceivers and cabling. The curve is useful for getting a quick idea of what is achievable and what is not, but it doesn't need to be considered a hard and fast rule.

    I hope this helps - let me know if you have any other questions.

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 30 points

    Max,

    Thank you very much! That was just what I needed to know.

    If there's a documentation department that could handle a request: at least for me increasing the size of that chart and drawing in the minor logarithmic gridlines would have been helpful for me and might be helpful for others as well.

    This is my first time using TI e2e and I am amazed at how quickly and thoroughly you responded. TI already had a lot of points in my book but it just got a lot more.

    Thank you,

    Alex

  • 0 in reply to Alex Suchko
    TI__Guru 62910 points
    Alex,

    Thank you! And I think you have some good feedback about the minor gridlines; it would make it a lot easier to decipher the log/log plot. I'll make a note to look into incorporating that next time we publish a revision to the datasheet.

    Max