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Re: RS485 over optical fiber cable

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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'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).
> 

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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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