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TIDA-00790: Cable length and bit rate

na na78
Guru 16770 points
Part Number: TIDA-00790
Other Parts Discussed in Thread: SN65HVD78, SN65HVD75, SN65HVD24

Hi

Our customer is referring TIDA-00790.

According to the table 1, SN65HVD78 would achieve 50Mbps at 150m cable length along with proper termination.

They are asking the following question.

If the bit rate cand be decreased to 5Mbps how long length of cable would be accepted for good communication?

Our customer want to achieve 5Mbps with 300m cable length.

BestRegards

  • 0
    TI__Guru 62910 points
    Hello,

    I just gave an explanation on RS-485's achievable bit rates for various cable lengths in your other post here:

    e2e.ti.com/.../2303436

    The cable loss is the most critical factor. Can you please let us know what this would be for your customer?

    Max
  • 0 in reply to Max Robertson
    Guru 16770 points
    Hi Max

    Thank you.

    They considers CAT5e cable.

    In their conventional system, they have been used SN65HVD75 in 640Kbps at 300m cable length. That has been worked well.

    But now 2.5Mbps is required, and the signal is degraded too much with 300m length.

    I think it is the best way if using equalization circuit makes it possible for SN65HVD75 to work in 2.5Mbps at 300m cable length.

    What do you think about it?

    BestRegards
  • 0 in reply to na na78
    TI__Guru 62910 points
    Yes, at that rate you are probably more limited by signal jitter than DC attenuation, so equalization is a possible a solution. While you are right that passive equalization could be designed (per the TIDA you mentioned), there is a limit to its effectiveness - there is no ability to add gain in a passive network, so you have to trade off signal loss for frequency response flatness.

    Given that, you may want to consider a device with active equalization like SN65HVD24. It is pin-to-pin with HVD75 except for the power supply voltage (5 V rather than 3.3 V), and would give significantly better performance over an extended distance. If the 5-V supply is not available, though, I do agree you could consider a passive solution.

    Max
  • 0 in reply to Max Robertson
    Guru 16770 points
    Hi Max

    Thank you for your reply.

    I had forgotten that full-duplex transceiver is required.
    SN65HVD24 can offer good performance, but it is half duplex device and it seems high class.

    Let me close this thread relevant to TIDA.
    Thank you for your cooperation.

    BestRegards