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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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#28292 — RS485 over optical fiber cable

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?

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#28293

marți, 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.

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#28296

Il 30/10/2018 15:26, raimond.dragomir@gmail.com ha scritto:
> marți, 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.

Do you use CAT5E cable and connect A and B to a couple?
What's about termination resistor?  In the past I used very small RS485 
links (under 100m) with low bitrate (maximum 38400bps), so I never used 
termination resistor.

I think I need it for 500m link.  Is the usual 120R ok?


> I also use small 5V:5V DC/DC converters on all nodes to isolate the RS485 part.

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#28297

miercuri, 31 octombrie 2018, 13:41:58 UTC+2, pozz a scris:
> Il 30/10/2018 15:26, raimond.dragomir@gmail.com ha scritto:
> > marți, 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.
> 
> Do you use CAT5E cable and connect A and B to a couple?
> What's about termination resistor?  In the past I used very small RS485 
> links (under 100m) with low bitrate (maximum 38400bps), so I never used 
> termination resistor.
> 
> I think I need it for 500m link.  Is the usual 120R ok?
> 
> 
> > I also use small 5V:5V DC/DC converters on all nodes to isolate the RS485 part.

Never used termination resistors either, even with networks up to 1km.
I also use only baudrates up to 38400bps, usually 9600.
I'm sure about these combinations:
1000m/9600 
500m/38400
All without termination resistors.

Never used CAT cables for RS485 but I heard it's possible. If I remember correctly, the termination for CAT5 should be 100R. 
I'm using "normal" multi-wire cables, not even twisted, but shielded. Connect the shield in only one place, to earth. If you don't know where you have a good earth, connect to the biggest metal enclosure which has an earth screw :-) Don't connect the shield to any electrical ground. The ground of the RS485 (C) is a floating ground.

With this opto-isolated RS485 system I never had headaches in very harsh environments.

Don't trust the saying that says the RS485 is a two wire connection. It's not. It's three wire. Make sure A,B,C is connected only to A,B,C. Treat the C as a signal wire, don't use the shield of the cable for it. Don't connect the C to anything else.

I hope this will help you.

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#28298

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.

-- 
Grant Edwards               grant.b.edwards        Yow! I think my career
                                  at               is ruined!
                              gmail.com            

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#28301

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).

JJS

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#28302

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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#28306

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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#28307

On 11/2/18 9:44 AM, pozz 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?
> 
> 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.
> 

You should have the signal ground. Without it, you can not control the
common mode voltage of the differential pairs. The receivers likely have
a small common mode bias current, and without some form of ground
return, the common mode bias currents will just charge up they parasitic
capacitance until you get a bias voltage high enough to change that bias
current, at which point the receiver is likely impaired in functionality
if not make the system inoperable.

There are some tricks you can do to fake the ground signal, (the
fail-safe termination can help a lot here), but you need to be REAL
careful about leakages to the outside world.

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#28311

On Fri, 2 Nov 2018 11:00:29 -0400, Richard Damon
<Richard@Damon-Family.org> wrote:

>On 11/2/18 9:44 AM, pozz 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?
>> 
>> 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.
>> 
>
>You should have the signal ground. Without it, you can not control the
>common mode voltage of the differential pairs. The receivers likely have
>a small common mode bias current, and without some form of ground
>return, the common mode bias currents will just charge up they parasitic
>capacitance until you get a bias voltage high enough to change that bias
>current, at which point the receiver is likely impaired in functionality
>if not make the system inoperable.

As long as

1,) There are floating power supplies (such as batteries) on each
transceiver
2.) There are "fail-safe" resistors from local Vcc and local Gnd
3.) There are at least one termination resistance between A/B at the
end of the bus.

The link works fine without the signal ground.  Each node is nicely
biased with local Vcc 1-3 V above common bus voltage and local Gnd 1-3
V below. 

This is OK as long as _all_ three_conditions are met.

The other question is what this common average voltage is, especially
if there is some external charge source. Grounding the termination
resistance midpoint to a real physical ground through a big resistor
_at_one_end_only_ will discharge any static buildup.  

Alternatively, if the cable has a common shield,  connect it to real
ground through a resistor at one end only to keep any static buildup
from A/B.

This works well in practice, some RS-422/485 converters (e.g. some
Westermo) do not even provide a signal ground (C) connector :-)



>
>There are some tricks you can do to fake the ground signal, (the
>fail-safe termination can help a lot here), but you need to be REAL
>careful about leakages to the outside world.

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#28308

vineri, 2 noiembrie 2018, 15:44:56 UTC+2, pozz a scris:
> 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?
> 
Don't avoid the signal ground. As I said before, the connection is NOT two wire. The problems start when you connect non-isolated grounds. If your devices are battery powered, they are already isolated, you should be happy.
Mains powered devices have this problem. Between sites/buildings the ground difference may be thousands of volts really at least in industrial fields. You risk to start fire as someone said it here previously.

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#28309

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.


-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

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#28312

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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#28313

On 02/11/2018 19:31, Tauno Voipio wrote:
> 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 Common Mode Range for RS-485 is -7V to +12V, i.e. +/-7V on top of 
the 0-5V signal range. A good receiver, or one that adheres to the 
standard should not insist on a midpoint of 2.5V

> The bias resistors are still necessary to handle an idle line.

Agreed.


-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

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#28315

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.

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. :-(



-- 
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

I have not yet begun to byte!

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#28316

On Fri, 02 Nov 2018 23:57:19 +0100, Stef
<stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:

>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.

The only case I can understand is power consumption especially in
battery powered systems. There is quite lot power dissipated in the
termination resistor and in drivers. This is a real problem with
RS-422 in which the transmitters are always active in idle Mark state.

In RS-485 the transmitter is active only when a specific node wants to
transmit,otherwise all transmitters are in tree-state.

Putting a big capacitor in series with the termination resistor
reducing idle consumption especially in RS-422 without hurting
transients,

>
>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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#28317

On 02/11/2018 23:18, upsidedown@downunder.com wrote:
> On Fri, 02 Nov 2018 23:57:19 +0100, Stef
> <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
> 
>> 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.
> 
> The only case I can understand is power consumption especially in
> battery powered systems. There is quite lot power dissipated in the
> termination resistor and in drivers. This is a real problem with
> RS-422 in which the transmitters are always active in idle Mark state.

Then why not use series termination as used in all low power systems 
I've ever come across?

-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

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#28318

On 11/3/18 3:05 PM, Mike Perkins wrote:
> On 02/11/2018 23:18, upsidedown@downunder.com wrote:
>> On Fri, 02 Nov 2018 23:57:19 +0100, Stef
>> <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
>>
>>> 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.
>>
>> The only case I can understand is power consumption especially in
>> battery powered systems. There is quite lot power dissipated in the
>> termination resistor and in drivers. This is a real problem with
>> RS-422 in which the transmitters are always active in idle Mark state.
> 
> Then why not use series termination as used in all low power systems
> I've ever come across?
> 

Series termination is really only appropriate for a point-to-point
system (one driver, one receiver) as it is based on the signal going to
half level until it reaches the far end when the reflection makes it
become full strength, and then the reflection comes back to the driving
end and the termination keeps it from bouncing back, so middle points
see 1/2 levels for a period until the reflection comes back.

One thing that can be done is to terminate with a series R-C network so
their is no idle current, but the active edge still sees the appropriate
impedance to avoid reflections. You may then add a much higher resistor
across the capacitor for the fail-safe bias network (with significantly
higher resistances than for a pure restive termination to provide a
clean DC bias in idle.

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#28319

On Saturday, November 3, 2018 at 5:36:33 PM UTC-4, Richard Damon wrote:
> On 11/3/18 3:05 PM, Mike Perkins wrote:
> > On 02/11/2018 23:18, upsidedown@downunder.com wrote:
> >> On Fri, 02 Nov 2018 23:57:19 +0100, Stef
> >> <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
> >>
> >>> 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.
> >>
> >> The only case I can understand is power consumption especially in
> >> battery powered systems. There is quite lot power dissipated in the
> >> termination resistor and in drivers. This is a real problem with
> >> RS-422 in which the transmitters are always active in idle Mark state.
> > 
> > Then why not use series termination as used in all low power systems
> > I've ever come across?
> > 
> 
> Series termination is really only appropriate for a point-to-point
> system (one driver, one receiver) as it is based on the signal going to
> half level until it reaches the far end when the reflection makes it
> become full strength, and then the reflection comes back to the driving
> end and the termination keeps it from bouncing back, so middle points
> see 1/2 levels for a period until the reflection comes back.

With parallel termination doesn't every point see a significantly lower level all the time?  The drivers aren't voltage sources, they still have significant series resistance, no? 

Rick C. 

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#28320

On Sat, 3 Nov 2018 19:49:15 -0700 (PDT),
gnuarm.deletethisbit@gmail.com wrote:

>On Saturday, November 3, 2018 at 5:36:33 PM UTC-4, Richard Damon wrote:
>> On 11/3/18 3:05 PM, Mike Perkins wrote:
>> > On 02/11/2018 23:18, upsidedown@downunder.com wrote:
>> >> On Fri, 02 Nov 2018 23:57:19 +0100, Stef
>> >> <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
>> >>
>> >>> 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.
>> >>
>> >> The only case I can understand is power consumption especially in
>> >> battery powered systems. There is quite lot power dissipated in the
>> >> termination resistor and in drivers. This is a real problem with
>> >> RS-422 in which the transmitters are always active in idle Mark state.
>> > 
>> > Then why not use series termination as used in all low power systems
>> > I've ever come across?
>> > 
>> 
>> Series termination is really only appropriate for a point-to-point
>> system (one driver, one receiver) as it is based on the signal going to
>> half level until it reaches the far end when the reflection makes it
>> become full strength, and then the reflection comes back to the driving
>> end and the termination keeps it from bouncing back, so middle points
>> see 1/2 levels for a period until the reflection comes back.
>
>With parallel termination doesn't every point see a significantly lower level all the time?  The drivers aren't voltage sources, they still have significant series resistance, no? 
>
>Rick C. 

Most RS-485 chips can deliver at least +/-20 mA, so there is +/-1 V
across the 100//100 ohm termination, Long wires with high resistance
will reduce this. The threshold is +/-0.2 V.

While the voltage is lower, significant current will flow and power
dissipated. This is a brute force method, but helps keeping the
interference out of the connection by improving signal-to-noise ratio.
 

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