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SN65HVD33: Data rate dependency

g.f.
Guru 15510 points
Part Number: SN65HVD33

Hi,

I have a question about SN65HVD33 RS-485 Tranceiver.

The maximum data rate of SN65HVD33 is 26Mbps.
I understood that data rate will depend to bus length.
If bus length are long that the data rate will be slower.

I want to know whether the data rate will also depend to the number of connected node.
Q1.
Does the data rate vary depending on the number of connected nodes?

Q2.
We are planning to connect 32 nodes to the bus.
In this case, is it possible to communicate at a maximum of 26 Mbps?
Or will it be much less than 26Mbps, for example about half data rate of 26Mbps?

best regards,
g.f.

  • 0
    TI__Guru 62910 points
    Hi g.f.,

    We'll get back to you tomorrow on this - sorry for the delay.

    Max
  • 0 in reply to Max Robertson
    TI__Mastermind 48215 points
    Hi g.f.,

    Apologies for the delay.

    Q1: Yes. Each node introduces load capacitance and resistance which can impact signal integrity. A very significant (and common) factor is stub-nodes causing reflections on the bus. The best way to avoid this is by daisy-chaining each node and limiting the lengths of any stubs as much as possible. The length of cable being used between nodes also introduces capacitance and reflections to the system.
    There is also a set maximum number of nodes based on DC leakage from finite input resistance that effects the system regardless of data rate. This shouldn't be the limiting factor with the number of nodes your considering though.

    Q2: The rated 26Mbps is most likely specified for a two-node system with minimal length between them. I would not expect a larger system to be able to sustain very reliable communications at this rate. Regarding how much the rate is effected by each node, I will be conducting tests on Monday on a similar device with variable loading conditions. I will update you on the trend that I find. Hopefully it will help you make some decisions in your design.

    In the meantime, let me know if you have any more questions.

    Regards,
    Eric
  • 0 in reply to Eric Schott1
    Guru 15510 points
    Hi Eric,

    Thank you for the reply and details.
    I understood that data rate depend to also the number of nodes.

    The test will be so great and helpful for us.
    Thank you so much. I will looking forward for the update from you.

    For now, I have no additional question.

    best regards,
    g.f.
  • 0 in reply to Eric Schott1
    Guru 15510 points
    Hi Eric,

    I'm sorry for disturbing you.
    Do you have any update about the test?

    best regards,
    g.f.
  • 0 in reply to g.f.
    TI__Mastermind 48215 points

    Hi g.f.,

    Sorry for the delay on this. I expect to have the results for the loading test by the end of this week. Thank you for your patience. 

    Regards,

    Eric

  • 0 in reply to Eric Schott1
    Guru 15510 points
    Hi Eric,

    Thank you for the reply.
    Okay, I understood.

    best regard,
    g.f.
  • 0 in reply to Eric Schott1
    Guru 15510 points
    Hi Eric,

    I'm sorry to keep bothering you.
    Have you finished the test?

    best regards,
    g.f.
  • 0 in reply to g.f.
    TI__Mastermind 25705 points

    g.f.

    So sorry for the delay of sending you the data. One more quick question, the 32 nodes are all RX in your application? In other words, is one TX (Z/Y pins) connected to 32 RX (A/B pins) on the bus?

    Regards,

    Hao

  • 0 in reply to Hao L
    Guru 15510 points
    Hi Hao,

    Thank you for the reply.

    Actualy this question was from my customer.
    Yes, one TX and 32 RX on the bus in their application.

    best regards,
    g.f.
  • 0 in reply to g.f.
    TI__Mastermind 25705 points

    g.f.,

    RX input has some resistance and small capacitance. Therefore at 26Mbps, 32 RX nodes would bring some voltage attenuation but not much jitter. Please see the attachment for lab measurement. Please let me know if you have any further questions.

    HVD33_load.pptx

    Regards,

    Hao