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THVD1424: Cable length vs. Data rate

Atsushi Takahashi
Expert 1050 points
Part Number: THVD1424
Other Parts Discussed in Thread: SN65HVD22

Tool/software:

Hi Team,

I have a question about SLLA628.
https://www.ti.com/lit/an/slla628/slla628.pdf

There exists a tradeoff between speed and maximum bus length.
If 5% jitter is acceptable, is my understanding correct that this device supports the following specs?

Max cable length is 200ft(60m) at 20Mbps.
Max cable length is 2000ft(600m) at 500kbps.
Max cable length is 4000ft(1200m) at 300kbps


Section 2.2.3 says that “further complicate the trade-off is the question of network topology”.
This product supports 256 nodes.
Are the above three specs of data rate vs. cable length met at 256 nodes?

Best Regards,

  • 0
    Guru 277650 points

    Typical RS-485 buses support these specs (regardless of what transceivers you are using).

    These values are specified for the maximul load, i.e., 256 nodes. But the remark about network topology does not talk about the number of nodes; it says that if you have stubs, then the jitter becomes worse.

  • 0
    TI__Guru 58260 points
    Atsushi Takahashi said:
    Max cable length is 200ft(60m) at 20Mbps.
    Max cable length is 2000ft(600m) at 500kbps.
    Max cable length is 4000ft(1200m) at 300kbps

    The scaling on this graph is actual logarithmic so I think some of the values you highlighted are slightly off. (Your drawings assume linear)

    300kbps is 4000ft (1.2km)

    500kbps is ~2500ft (760km)

    20Mbps is around 200ft (60km)

    Atsushi Takahashi said:

    Section 2.2.3 says that “further complicate the trade-off is the question of network topology”.
    This product supports 256 nodes.
    Are the above three specs of data rate vs. cable length met at 256 nodes?

    No.

    I believe this graph comes from the RS422 standard (not 100% sure) but I assume that the graph uses the assumption that the bus is terminated and there are no additional nodes on the bus. With additional loading and stubs or bad design practices you may end up getting less distance. If you used a cable without impedance controlled wires then this distance wouldn't be possible. Or if you have a bunch of parallel lines/stubs on the bus then this would shorten the distance (like if you used a star network or a ring network). The best way to achieve this distance with the speed is to use a daisy chain and properly terminated lines with impedance controlled cables.

    -Bobby

  • 0 in reply to BOBBY
    Expert 1050 points

    Hi Bobby-san,

    Thank you for your reply.

    Is this concept the same not only for THVD1424 but also for other TI RS-485 transceivers?
    The customer is considering replacing SN65HVD22 with THVD1424, and would like to know if it covers the specifications(cable length, data rate and node) of SN65HVD22.

  • +1 in reply to Atsushi Takahashi
    TI__Guru 58260 points

    Hi Atsushi,

    Yes, that correct. The graph we keep referring to is basically a concept of how far and fast RS422/485 can support. So it would apply to other RS485 transceivers as well. The SN65HVD22 doesn't have anything special in it's receiver input like the HVD23 and HVD24 do (equalizer circuit) so it's distance/speed should be comparable to THVD1424. From a datasheet perspective, the THVD1424 looks like it has a higher VoD and faster rise/fall times on it's output so it should perform similar or better at driving a cable. 

    -Bobby