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RS485 over optical fiber cable

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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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#28321

Il 02/11/2018 23:57, Stef ha scritto:
> On 2018-11-02 Tauno Voipio wrote in comp.arch.embedded:
> [...]
>> The bias resistors are still necessary to handle an idle line.
> 
> With "fail safe" receivers, a termination resistor alone will
> keep the differential voltage low enough for the receiver to see
> 'idle'. But if ypou can ad bias, why not do it?
> 
> 
> Why is Pozz trying to do this without termination?
> On RS485, use termination! Never had trouble with termination,
> have had trouble without.

I try to clarify my questions on termination resistors.

I can really use them. The higher power dissipation is interesting in my 
battery-powered application, but I think I will have enough energy. So 
this isn't a problem.

I will use a termination resistor on both sides. My concerns were on the 
resistor correct value. If I need a termination, I need to use a correct 
resistor value. Is it critical?

The other problem with signal ground, I know it is better to connect it 
together with A and B. However it would be much more simple and cheap to 
use a simple couple instead of a full CAT5E cable.


> Just remembered an occasion where I had to source a resistor in a
> remote location with no electronics shop anywhere nearby. And then
> climb a 70m ladder to install it. Then you really wonder why they
> originally did not install a termination resistor. :-(

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#28323

On 11/5/18 3:40 AM, pozz wrote:
> Il 02/11/2018 23:57, Stef ha scritto:
>> On 2018-11-02 Tauno Voipio wrote in comp.arch.embedded:
>> [...]
>>> The bias resistors are still necessary to handle an idle line.
>>
>> With "fail safe" receivers, a termination resistor alone will
>> keep the differential voltage low enough for the receiver to see
>> 'idle'. But if ypou can ad bias, why not do it?
>>
>>
>> Why is Pozz trying to do this without termination?
>> On RS485, use termination! Never had trouble with termination,
>> have had trouble without.
> 
> I try to clarify my questions on termination resistors.
> 
> I can really use them. The higher power dissipation is interesting in my
> battery-powered application, but I think I will have enough energy. So
> this isn't a problem.
> 
> I will use a termination resistor on both sides. My concerns were on the
> resistor correct value. If I need a termination, I need to use a correct
> resistor value. Is it critical?

Most twisted pairs are around 100 Ohms, so you use a terminating
resistor near there. Being slightly off isn't normally an issue, it just
says that you get a bit of a reflection. RS485 isn't that high speed (or
fast edge rate) so that makes things even a bit less critical, and ring
tolerances higher.
> 
> The other problem with signal ground, I know it is better to connect it
> together with A and B. However it would be much more simple and cheap to
> use a simple couple instead of a full CAT5E cable.
> 
> 

As has been said, the key requirement for the ground is to keep the
common mode voltage in range. This means that when using it for long
distance links you need to pay attention to grounding issues, and often
need to worry about isolation to avoid ground currents.

>> Just remembered an occasion where I had to source a resistor in a
>> remote location with no electronics shop anywhere nearby. And then
>> climb a 70m ladder to install it. Then you really wonder why they
>> originally did not install a termination resistor. :-(

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#28324

On 11/5/18 12:40 AM, pozz wrote:
> Il 02/11/2018 23:57, Stef ha scritto:
>> On 2018-11-02 Tauno Voipio wrote in comp.arch.embedded:
>> [...]
>>> The bias resistors are still necessary to handle an idle line.
>>
>> With "fail safe" receivers, a termination resistor alone will
>> keep the differential voltage low enough for the receiver to see
>> 'idle'. But if ypou can ad bias, why not do it?
>>
>>
>> Why is Pozz trying to do this without termination?
>> On RS485, use termination! Never had trouble with termination,
>> have had trouble without.
> 
> I try to clarify my questions on termination resistors.
> 
> I can really use them. The higher power dissipation is interesting in my 
> battery-powered application, but I think I will have enough energy. So 
> this isn't a problem.
> 
> I will use a termination resistor on both sides. My concerns were on the 
> resistor correct value. If I need a termination, I need to use a correct 
> resistor value. Is it critical?
> 
> The other problem with signal ground, I know it is better to connect it 
> together with A and B. However it would be much more simple and cheap to 
> use a simple couple instead of a full CAT5E cable.
> 
> 
>> Just remembered an occasion where I had to source a resistor in a
>> remote location with no electronics shop anywhere nearby. And then
>> climb a 70m ladder to install it. Then you really wonder why they
>> originally did not install a termination resistor. :-(

Depending on the application, you may also be able to save power by 
disabling the transmitter when you're not transmitting.  You'll need to 
think about the implications for the receiver: the failsafe logic, etc. 
But if you're only transmitting 1% of the time, you only need to be 
dumping current into those termination resistors 1% of the time.

-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.

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#28310

On 2018-11-02, pozz <pozzugno@gmail.com> wrote:

> 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?

Yes, it's "safe" (nothing's going to melt or burst into flames and
kill people).  No, I wouldn't depend on it working reliably (though it
probably will).

> 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.

There will be some sort of current leakage that will cause the the
signal ground to "float" to some voltage relative to the two signal
wires, but it's pretty difficult to predict exactly what happens.

It's far easier to understand what happens when you _do_ have a signal
ground connection.

-- 
Grant Edwards               grant.b.edwards        Yow! I'm using my X-RAY
                                  at               VISION to obtain a rare
                              gmail.com            glimpse of the INNER
                                                   WORKINGS of this POTATO!!

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#28314

On Fri, 2 Nov 2018 18:21:55 +0000 (UTC), Grant Edwards
<invalid@invalid.invalid> wrote:

>On 2018-11-02, pozz <pozzugno@gmail.com> wrote:
>
>> 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?
>
>Yes, it's "safe" (nothing's going to melt or burst into flames and
>kill people).  No, I wouldn't depend on it working reliably (though it
>probably will).
>
>> 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.
>
>There will be some sort of current leakage that will cause the the
>signal ground to "float" to some voltage relative to the two signal
>wires, but it's pretty difficult to predict exactly what happens.

In 230/400 V mains environment, with data lines running in parallel
with mains wires for long distances, the worst that would happen is
the data bus swinging between +320 V and -320 V due to capacitive
coupling.As long as the data isolation voltage is larger than that
swing, who cares.

In practice the resultant capacitive field from a mains _cable_, is
much less since much of the external field is canceled by the neutral
or other phases and the coupled voltages are much  lower.

>It's far easier to understand what happens when you _do_ have a signal
>ground connection.

Does the century old unipolar 20 mA current loop need a separate
ground wire in addition to one conductor carrying current to the
receiver and an other back from the receiver ? No it doesn't, look for
instance at the telephone connector.

RS-422/485 with a single 100 ohm terminator resistor and minimum
+/-200 mV swing, is essentially a +/- 2mA bipolar current loop. In
practice, the loop current is closer to +/- 20 mA and hence the
voltage swing larger. Why would a current loop need a ground voltage
reference ?

If bipolar transistors are used to sense the voltage across the
termination resistance, the issue is providing some base current to
the bipolar transistors.However, with "fail-safe" resistors installed,
there is a current flowing

* from Vbat+ 
* through upper "fail-safe" resistor 
* to top of termination resistor
* through termination resistor
* from bottom of termination resistor
* through other "fail-safe" resistor
* into Vbat-

The transistor bases can rob enough current to bias the input
transistors. Do no need for a separate ground reference wire.

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#28305

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

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#28300

On Tue, 30 Oct 2018 07:26:31 -0700 (PDT), raimond.dragomir@gmail.com
wrote:

>marÈ i, 30 octombrie 2018, 16:00:06 UTC+2, pozz a scris:
>> I'd like to replace an electrical cable with a fiber cable, mainly for 
>> electromagnetic noise immunity and long distance (I need to reach 500m).
>> The bitrate will be very low: 9600bps is ok.
>> I need to keep the cost low as possible.
>> 
>> I never used fiber cables before. In similar applications I used RS485 
>> transceivers connected to the UART peripheral of an MCU.
>> 
>> I think I need a fiber optic cable transceiver, but I don't know what to 
>> search and how to interconnect with an MCU.
>> 
>> Any help?
>
>9600bps, 500m, RS485.
>It's all you need.
>Probably you also need galvanic isolation. Use optocouplers.
>Use three wires, A,B,C. C is the common ground for ONLY RS485 communication, isolated on ALL ends.
>I also use small 5V:5V DC/DC converters on all nodes to isolate the RS485 part.

I second this, use optocouplers. 40 years ago we used this to drive
printers like the Centronics 306. With a current loop system to carry
1200 bd over (if I remember correctly) max >4 km with dual twisted
pair low-capacity cable, screened, in a very noisy environment. Using
dual optocouplers in the peripherals, one wire pair carried data to
the peripheral (driving a led in the optocoupler there), the other
pair sensed whether the peripheral was driving its led (for
offline/busy). Worked like a charm. Of course the screen was only
connected at one side (computer).

Mat Nieuwenhoven

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#28303

On Tue, 30 Oct 2018 07:26:31 -0700 (PDT), raimond.dragomir@gmail.com
wrote:

>mar?i, 30 octombrie 2018, 16:00:06 UTC+2, pozz a scris:
>> I'd like to replace an electrical cable with a fiber cable, mainly for 
>> electromagnetic noise immunity and long distance (I need to reach 500m).
>> The bitrate will be very low: 9600bps is ok.
>> I need to keep the cost low as possible.

If the stations are in different buildings with different mains feeds
and/or separate (lightning) grounding electrode, IMHO the only option
is using fibers in order to survive lightning hits of one  building
(not hitting data cables, only mains overvoltages). 

Simple galvanic isolation (500-2500 V) is definitely not enough !
 
>> 
>> I never used fiber cables before. In similar applications I used RS485 
>> transceivers connected to the UART peripheral of an MCU.
>> 
>> I think I need a fiber optic cable transceiver, but I don't know what to 
>> search and how to interconnect with an MCU.
>> 
>> Any help?
>
>9600bps, 500m, RS485.
>It's all you need.
>Probably you also need galvanic isolation. Use optocouplers.
>Use three wires, A,B,C. C is the common ground for ONLY RS485 communication, isolated on ALL ends.
>I also use small 5V:5V DC/DC converters on all nodes to isolate the RS485 part.

Optoisolate 500 m within the same building is usually OK, with
different buildings, only fibers should be used.

"Fail safe" terminations (quotes are in the standard text :-) i.e.
pull up/down resistors on each node and 100-120 ohm termination
resistors at both end of the bus should be used. The resistors are
very important, since a 2 wire system goes three-state between message
exchanges.

If two pairs are available, use properly terminated RS-422 for
point-to-point or 4 wire RS-485 in which case the master Tx pair the
Tx is always on (improving noise immunity) or in multidrop slaves the
other pair is actively driven only when the addressed slave wants to
transmit.

For 500 m, forget about using Rx/Tx transceiver common mode (+12/-7 V)
range only without isolation.

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#28294

On 31/10/2018 1:00 am, pozz wrote:
> I'd like to replace an electrical cable with a fiber cable, mainly for 
> electromagnetic noise immunity and long distance (I need to reach 500m).
> The bitrate will be very low: 9600bps is ok.
> I need to keep the cost low as possible.
> 
> I never used fiber cables before. In similar applications I used RS485 
> transceivers connected to the UART peripheral of an MCU.
> 
> I think I need a fiber optic cable transceiver, but I don't know what to 
> search and how to interconnect with an MCU.
> 
> Any help?

Depending on your idea of 'low cost' we've used these with great 
success: <https://www.moxa.com/product/TCF-142.htm>

But for 500m and cheap, I'd also go with a solution like Raimonds (other 
reply).

--
Cheers,

Chris.

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#28304

On Wed, 31 Oct 2018 12:16:50 +1100, Chris <c@b.a> wrote:

>On 31/10/2018 1:00 am, pozz wrote:
>> I'd like to replace an electrical cable with a fiber cable, mainly for 
>> electromagnetic noise immunity and long distance (I need to reach 500m).
>> The bitrate will be very low: 9600bps is ok.
>> I need to keep the cost low as possible.
>> 
>> I never used fiber cables before. In similar applications I used RS485 
>> transceivers connected to the UART peripheral of an MCU.
>> 
>> I think I need a fiber optic cable transceiver, but I don't know what to 
>> search and how to interconnect with an MCU.
>> 
>> Any help?
>
>Depending on your idea of 'low cost' we've used these with great 
>success: <https://www.moxa.com/product/TCF-142.htm>

Since standard Ethernet components are cheap these days, one option to
consider is running the 500 m with suitable ethernet fiber components
and at the remote end install Ethernet/RS-485 converter. If there is
only one station at the remote site, an even cheaper Ethernet/RS-232
converter could be used.

>
>But for 500m and cheap, I'd also go with a solution like Raimonds (other 
>reply).

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#28299

On Tue, 30 Oct 2018 15:00:00 +0100, pozz <pozzugno@gmail.com> wrote:

>I'd like to replace an electrical cable with a fiber cable, mainly for 
>electromagnetic noise immunity and long distance (I need to reach 500m).
>The bitrate will be very low: 9600bps is ok.
>I need to keep the cost low as possible.
>
>I never used fiber cables before. In similar applications I used RS485 
>transceivers connected to the UART peripheral of an MCU.
>
>I think I need a fiber optic cable transceiver, but I don't know what to 
>search and how to interconnect with an MCU.
>
>Any help?


External converters of that sort used to be fairly common, but rather
less so these days.  If you have a handful of installations with
problems they might make sense, but are pricey otherwise.  Black Box
at least still lists one in their catalog:

https://www.blackbox.com/en-us/store/Detail.aspx/Async-RS232-RS422-RS-485-Extender-over-Fiber---DB25-Female-ST/ME610A-US

Another vendor:

http://datainterfaces.com/FRM220-SERIAL-ST002.aspx

If you're looking to embed this into your device, then you really
ought to back up to the UART, and then put the fiber transceiver on
the UART's RX and TX.  Something along the lines of a Broadcom
AFBR-59F2Z (just one example) would work, although that particular
device has LVDS RX/TX inputs/outputs.  Converting that to the single
ended/common mode signaling from the typical UART is not hard, but
will require some work, you might be able to find a more directly
usable device.

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#28339

On Wednesday, October 31, 2018 at 12:00:06 AM UTC+10, pozz wrote:
> I'd like to replace an electrical cable with a fiber cable, mainly for 
> electromagnetic noise immunity and long distance (I need to reach 500m).
> The bitrate will be very low: 9600bps is ok.
> I need to keep the cost low as possible.
> 
> I never used fiber cables before. In similar applications I used RS485 
> transceivers connected to the UART peripheral of an MCU.
> 
> I think I need a fiber optic cable transceiver, but I don't know what to 
> search and how to interconnect with an MCU.
> 
> Any help?

I have converted from RS232/RS422/RS485 to single-mode fiber over tens of kilometers. In fact I had that much bandwidth for RS232 that I would just create virtual wires from the 8 signals so baud rate etc wasn't important. What changed on one end would change on the other. For megabaud RS485 rates though I would buffer the data and send it in high speed packets to reduce latency and also allows for control signals. 

However for short distances up to a 1000m or so I would just use TP with proper termination which is easy if it is essentially point-to-point. Most of my systems though are multidrop and I have even used 4-core phone cable to carry DC and 485 to nodes inside a building. In one of my recent designs I had about 40 nodes connected over about 15m  within a large cabinet with IDC cable and with the 485 signals on the two inner pairs, communicating at 2M baud. Despite the huge amounts of electrical noise it worked perfectly.

I am really surprised though to find that in 2018 people are still talking about RS485 "bias" resistors. Doesn't anyone use fail-safe type 485 chips? I use many different ones such as 65HVD75 or VP3082/3088 etc and these provide failsafe in both line open and line short conditions without the need for bias resistors. They are also 1/4 or 1/8 load and also have much more sensitive receivers that work down to an acceptable 80mV hysteresis. There are also other types that have huge common mode range. Besides many multidrop systems have problems because techs add termination and bias to nodes that "are missing it".    

But I'm in the two wire camp and I am fully aware of the technician's thinking that you "need a ground" and the logic that says there could be huge ground differentials also ignores the fact that if this is so, then connecting grounds/earths across these systems would and does result in huge noisy ground currents through the same cable conveying the signal, and so the systems better have a very good earth or none at all (isolated).

Isolation (solid-state) and 2-wire is better but over long distances you need lightning protection too, and I use combinations of transzorbs, ferrite beads, polyfuses, and miniature gas discharge tube protection and in these cases there is not a ground, but there is a very good earth.

Getting back to low speed RS485 and just how much you can get away with. I once ran RS485 at 19,200 baud multidrop over many branches of figure eight fire proof cable up and down lift wells over 26 floors and simply relied upon blindly repeating check-summed packets three times so that the master could address all lifts or groups of lifts simultaneously. Somehow it just worked and kept on working :) 

So no, you don't need fiber for 500m, just protect and isolate in hazardous and noisy environments. You can even get away with good optos at such a low speed and just use those tiny 4-pin isolated DC-DC converter modules to power the RS485 chip. You don't have to change anything with the MCU and UART methods. I prefer using solid-state isolators though such as the ADuM3211ARZ as they are much easier to use.

BTW, without the third wire the current flow through the differential receiver's inputs and through its ground is its reference so that one input is greater or less than the other. Think of the inputs as being resistors connected to the receiver's ground. Try connected a small battery across the receiver terminals and then swapping it to change the signal, none of which requires a third wire (for the signal). LTSpice this if you are in doubt.

Cheers,
Peter

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#28340

On Mon, 12 Nov 2018 19:57:18 -0800 (PST), peter jakacki
<peterjakacki@gmail.com> wrote:

>Isolation (solid-state) and 2-wire is better but over long distances you need lightning protection too, and I use combinations of transzorbs, ferrite beads, polyfuses, and miniature gas discharge tube protection and in these cases there is not a ground, but there is a very good earth.

In the context of lightning protection, what is a good earth ?

With two buildings with separate mains connections and individual
earthing electrodes, you do not even need a direct lightning hit, just
a hit to the mains leads and some arc suppressions to earth at mains
entry and you have several kA flowing into the earth. With earthing
resistance as low as 1 ohm, you will have a ground potential  bounce
of several kV in one building. I very much doubt that ordinary
isolation and some protection will handle this potential difference.

When connecting two buildings with individual earthing electrodes, I
would definitively use some fiber.

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