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SN65HVD3082E: Communication length

Wesley Huang
Expert 1030 points
Part Number: SN65HVD3082E

HI Team,

 

My system use 3082E for 485 communication whose speed is 115kpbs. When I increase the communication distance to approximately 110m, the communication data is received incorrectly.

  1. Here is my schematic, could you please help double check with this?

        2. Do you have a application note or formula which is about how to roughly calculate the maximum communication distance that 3082E can reach at a specific speed.

 

Best regards

Zhihong Huangs

  • 0
    TI__Guru 62910 points

    Hi Zhihong,

    You can find some references on achievable transmission distances for RS-485 in these application notes:

    http://www.ti.com/lit/an/slla375/slla375.pdf

    https://www.ti.com/lit/an/slla431/slla431.pdf

    In this case there are a few factors that could potentially limit the performance.  One is the series resistance of J4/J5, which combined with the increased series resistance of the longer cable may be resulting in too much attenuation of the signaling.  Can you please provide the part number for these components so that we can check it?  Another potential factor is use of termination resistances that are higher than the characteristic impedance of the cabling (which is typically ~120 Ohms for RS-485).  This would result in reflections occurring at the end of the line, and at different cable lengths the phase of this reflection at each receiver will vary.  Perhaps at 110 m the phase delay of the reflection becomes problematic enough to the signal integrity that bit errors occur.  Another potential factor would be the AC losses of the cabling resulting in timing jitter, although unless it is a very lossy cable typically these effects would not be seen at 115 kbps / 110 m.

    The best way to know what may be limiting the distance is by probing the A and B signal waveforms at the receiver as well as the corresponding R output.  Based on this, we can make a more educated guess as to why communication is not working.

    Max

  • 0 in reply to Max Robertson
    Expert 1030 points

    HI Max,

     

    Thanks for yout reply. These are very good notes.

    i will probe  the waveform later for you to make a more educated guess.

    Best regards

    Zhihong Huangs

  • 0 in reply to Wesley Huang
    Expert 1030 points

    HI Max,

    Could you please help me double check with my schematic? 

    Best regards

    Zhihong Huangs

  • 0 in reply to Wesley Huang
    TI__Guru 62910 points

    Hi Zhihong,

    Most of my concerns regarding the schematic were brought up in my initial post.  After reviewing one more time I did have one more major concern, though: it looks like the VCC voltage is 3.3 V, while the SN65HVD3082E is designed for 5-V applications and requires a minimum VCC level of 4.5 V.

    Max