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Groups > comp.arch.embedded > #28312
| From | Tauno Voipio <tauno.voipio@notused.fi.invalid> |
|---|---|
| Newsgroups | comp.arch.embedded |
| Subject | Re: RS485 over optical fiber cable |
| Date | 2018-11-02 21:31 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <pri8lu$ks7$1@dont-email.me> (permalink) |
| References | (3 earlier) <prccn8$spf$1@reader1.panix.com> <prcljq$9v1$1@gioia.aioe.org> <prcmch$2uj$1@reader1.panix.com> <prhkck$e2s$1@dont-email.me> <prhq40$63i$1@news2.open-news-network.org> |
On 2.11.18 17:22, Mike Perkins wrote: > On 02/11/2018 13:44, pozz wrote: >> Il 31/10/2018 17:48, Grant Edwards ha scritto: >>> 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. >> >> Yes, bias resistors are needed, even if modern transceivers seem to >> work well even without them (they are fail-safe even when all the >> drivers on the bus are disabled). >> >> >>> 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 will have two battery-powered devices, connected by a RS485 link. In >> this case, both devices are isolated from mains and Earth. >> Do you think it's safe to avoid signal ground connection between the >> two devices in this situation? >> >> It's difficult to me to understand what happens to the common-mode >> voltage at the receiver, if its power supply (battery) is isolated >> from the rest of the world. > > I might consider using two resistors in series as the 'terminating > resistor' and to use the mid point as the receiver's ground. The receiver supposes that the midpoint is in the middle between the supplies to it (typically at 2.5 V). The differential signal is one line at 5 V and the other at 0 V. The bias resistors are still necessary to handle an idle line. -- -TV
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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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