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| From | Grant Edwards <invalid@invalid.invalid> |
| Newsgroups | comp.arch.embedded |
| Subject | Re: RS485 over optical fiber cable |
| Date | Wed, 31 Oct 2018 16:48:17 +0000 (UTC) |
| Organization | PANIX Public Access Internet and UNIX, NYC |
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On 2018-10-31, John Speth <johnspeth@yahoo.com> wrote:
> 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).
And don't forget the _bias_ resistors to make sure that the line idles
in the proper state when no drivers are enabled.
And yes, a signal ground connection is also _required_. Even though
the signaling is differential, a signal ground is required to make
sure you don't violate the common-mode voltage specs on the
transceivers.
I've been dealing with customer RS485/RS422 issues for almost 30
years, and they're almost due to missing bias/terminating resistors or
missing signal ground.
You can connect signal ground to Earth ground if you want, but
_only_at_one_point_. Breaking that rule can start fires (yes, I've
seen the melted, charred results of connecting signal ground or cable
shield to Earth at to different points).
--
Grant Edwards grant.b.edwards Yow! Sometime in 1993
at NANCY SINATRA will lead a
gmail.com BLOODLESS COUP on GUAM!!
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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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