Re: RS485 over optical fiber cable

From	pozz <pozzugno@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Re: RS485 over optical fiber cable
Date	2018-11-02 14:36 +0100
Organization	A noiseless patient Spider
Message-ID	<prhjtk$b71$1@dont-email.me> (permalink)
References	<pr9o50$m9b$1@dont-email.me> <7686c6c0-7f2a-49b9-94d2-da49b7ab32ef@googlegroups.com> <prc4e3$chj$1@dont-email.me> <prccn8$spf$1@reader1.panix.com> <prcljq$9v1$1@gioia.aioe.org>

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Il 31/10/2018 17:35, John Speth ha scritto:
> On 10/31/2018 7:03 AM, Grant Edwards wrote:
>> On 2018-10-31, pozz <pozzugno@gmail.com> wrote:
>>
>>> I think I need it for 500m link.  Is the usual 120R ok?
>>
>> The terminating resistor value is chosen to match the cable's
>> impedance.
> 
> I have no experience with long wire RS485 applications.  I find it hard 
> to believe that an appropriately sized resistor is not on the top of the 
> list of RS485 best practices.  I have solved so many "long" wire 
> problems by just adding termination resistors (in my case long wires are 
> actually short but appear long due to edge speeds).

The model of the link can be a simple resistor or a transmission line.

The termination resistor is needed if the link can't be modelized as a 
simple resistor, but as a transmission line.
So the main question is: when the link can't be modeled as a simple 
resistor?

For what I understood [1], it depends on two factors: the length of the 
link *and* the speed of signal (baudrate).
Assuming 60% speed of light, 1km propagation delay is only 5us. At 
9600bps, one bit time is 104us, 20 times higher. I don't think 
termination resistor, even useful, can increase performance.

In my experience with many "small" RS485 networks, I never used 
termination resistors. When I tried to solve some problems with 
termination resistor, I never had good results.

[1] http://www.diit.unict.it/users/scava/dispense/II/RS485.pdf

Thread

RS485 over optical fiber cable pozz <pozzugno@gmail.com> - 2018-10-30 15:00 +0100
  Re: RS485 over optical fiber cable raimond.dragomir@gmail.com - 2018-10-30 07:26 -0700
    Re: RS485 over optical fiber cable pozz <pozzugno@gmail.com> - 2018-10-31 12:41 +0100
      Re: RS485 over optical fiber cable raimond.dragomir@gmail.com - 2018-10-31 05:55 -0700
      Re: RS485 over optical fiber cable Grant Edwards <invalid@invalid.invalid> - 2018-10-31 14:03 +0000
        Re: RS485 over optical fiber cable John Speth <johnspeth@yahoo.com> - 2018-10-31 09:35 -0700
          Re: RS485 over optical fiber cable Grant Edwards <invalid@invalid.invalid> - 2018-10-31 16:48 +0000
            Re: RS485 over optical fiber cable pozz <pozzugno@gmail.com> - 2018-11-02 14:44 +0100
              Re: RS485 over optical fiber cable Richard Damon <Richard@Damon-Family.org> - 2018-11-02 11:00 -0400
                Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-02 20:42 +0200
              Re: RS485 over optical fiber cable raimond.dragomir@gmail.com - 2018-11-02 08:18 -0700
              Re: RS485 over optical fiber cable Mike Perkins <spam@spam.com> - 2018-11-02 15:22 +0000
                Re: RS485 over optical fiber cable Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-11-02 21:31 +0200
                Re: RS485 over optical fiber cable Mike Perkins <spam@spam.com> - 2018-11-02 20:12 +0000
                Re: RS485 over optical fiber cable Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2018-11-02 23:57 +0100
                Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-03 01:18 +0200
                Re: RS485 over optical fiber cable Mike Perkins <spam@spam.com> - 2018-11-03 19:05 +0000
                Re: RS485 over optical fiber cable Richard Damon <Richard@Damon-Family.org> - 2018-11-03 17:36 -0400
                Re: RS485 over optical fiber cable gnuarm.deletethisbit@gmail.com - 2018-11-03 19:49 -0700
                Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-04 07:50 +0200
                Re: RS485 over optical fiber cable pozz <pozzugno@gmail.com> - 2018-11-05 09:40 +0100
                Re: RS485 over optical fiber cable Richard Damon <Richard@Damon-Family.org> - 2018-11-05 06:45 -0500
                Re: RS485 over optical fiber cable Rob Gaddi <rgaddi@highlandtechnology.invalid> - 2018-11-05 09:12 -0800
              Re: RS485 over optical fiber cable Grant Edwards <invalid@invalid.invalid> - 2018-11-02 18:21 +0000
                Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-03 00:02 +0200
          Re: RS485 over optical fiber cable pozz <pozzugno@gmail.com> - 2018-11-02 14:36 +0100
    Re: RS485 over optical fiber cable "Mat Nieuwenhoven" <mnieuw@zap.a2000.nl> - 2018-10-31 16:32 +0100
    Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-10-31 22:41 +0200
  Re: RS485 over optical fiber cable Chris <c@b.a> - 2018-10-31 12:16 +1100
    Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-02 06:52 +0200
  Re: RS485 over optical fiber cable Robert Wessel <robertwessel2@yahoo.com> - 2018-10-31 09:11 -0500
  Re: RS485 over optical fiber cable peter jakacki <peterjakacki@gmail.com> - 2018-11-12 19:57 -0800
    Re: RS485 over optical fiber cable upsidedown@downunder.com - 2018-11-13 08:09 +0200

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