• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 199 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.

SN65HVD23: Is there a device like this one that allows more nodes?

adsfsadfadsfda
Prodigy 90 points
Part Number: SN65HVD23
Other Parts Discussed in Thread: SN65HVD24, THVD1520, THVD1550, THVD1420

I'm trying to find an RS485 half-duplex transceiver that can handle over 100 nodes at a distance of over 100 meters and a speed over 10 Mbps (20-30 Mbps would be even better). I know that this is not easy and it depends on a lot of factors, such as cable quality.

I found the SN65HVD23, which is optimized for signaling around 25 Mbps with cable lengths of up to 200 meters, but it only allows 64 nodes.

The SN65HVD24, which is optimized for signaling around 5 Mbps with cable lengths up to 500 meters, and allows 256 nodes. However, the data rate is a bit lower that necessary.

Without carefully looking through every single datasheet, I'm having difficulty finding devices that have features for extending range, for example receiver equalization or preemphasis on the transmitter.

Does anyone have some suggested parts to look at?

  • 0
    Guru 277220 points

    There are not many parts with active filtering.

    Use modern transceivers with strong outputs like the THVD1520/THVD1550.

    The data rate is limited by the jitter introduced by the long cable. You might be able to improve the signal quality with common-mode chokes, but it also matters how jitter-tolerant you receiver logic is.

  • +1
    TI__Mastermind 46607 points

    Hi,

    Sorry for the delay on the response.

    The SN65HVD23/4 are our only devices with RX equalization network to help reduce jitter on longer bus lengths. 

    The chart that Clemens showed is the general one we suggest to follow (its assuming 120 Ohm 22AWG twisted pair - unshielded cabling - as this is a pretty ideal cable setup).

    The THVD1520 (or if you want enhanced ESD protection + wider supply range the THVD1420 is also a good option) can run at the correct data rate and you should be able run 100m up to 256 nodes - assuming you can handle some additional jitter to the data-signal; if you can use as close to the recommended wire as possible you could be seeing most likely 2% to 3% additive jitter from the bus. This is most likely the best option - unless you can swing the 3Mbps of the SN65HVD24 (which I understand most likely isn't an option - but that would be the next choice). I wouldn't go with the THVD1550 purely because it will degrade your max unterminated stub length - which  for your system there are 98 unterminated stubs - at 50Mbps you generally are looking at ~ 1/2 to 2/3 of a meter of unterminated stub length before it starts to cause issues. 

    One important note is that with 100 nodes the network topology and configuration are very important. While not defined in RS-485 standard specifically - we do recommend to use daisy chaining to keep unterminated stub length minimized -  

    (please use c in m/s for L in terms of max length with meter units - this slide was originally shown to a US based audience - but the basic equation is the same)

    v * c - is a cable parameter usually denoted as 1 parameter (it also is called Vp for phase velocity) - many cables will just a percentage of this value - like 78% - so v * c = 78% * c in that case. 

    If you violate this - the stubs will act as parallel transmission lines which is going to create reflections and potential signal quality issues that result in a failure. 

    If you have a non-ideal setup - generally you will need to add an additional line driver for every node and you would terminate every single node. This is usually not an ideal situation because it will drive up cost and complexity of the system in the signal chain and potentially the power tree as well. So I'd strongly suggest minimizing the unterminated stub lengths as much as possible.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson