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Re: RS485 is bidirectional does it mean it is fullduplex?

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Sorry, never heard that phrase before.  I assuem "frame" and
"chassis" were the same.  In the installations I've dealt with
the RS-485 common is certainly not chassis ground on either end.

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And you've got to somehow guarantee that the recievers common
mode DC voltage is within spec.  If the only DC connections to
the outside world are the A/B signal lines, how is that
accomplished?

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Are you talking about "inducing" a DC voltage?

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What "noise"?  I'm talking about controlling common-mode DC
level difference between the RS-485 transmitter and receiver.

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I think the spec is 10km for decent twisted pair and low baud
rates (<1M).

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Cable an electrician installed.

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The shield may be grounded at one end or the other, but the
RS-485 common is not.

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What are "frame grounds" and what do they have to do with the
RS-485 bus???

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You keep talking about "frame grounds" and earth and stuff.

The RS-485 systems I'm talking about are all optically isolated
from frame, chassis, and earth.  If you don't connect the RS-485
commons together with the cable, then you end up with
common-mode voltages out of spec.  Study all you want, that's
what happens in practice.

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I don't care what you do with the cable shield, and frame
grounds and chassis grounds, but they aren't connected to
RS-485 common.

--
Grant Edwards                   grante             Yow!  Someone is DROOLING
                                  at               on my collar!!
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Re: RS485 is bidirectional does it mean it is fullduplex?
May I summary this ?

I think RS485 problem is 2 :
 *) Noise over long distance cable and
 *) Faulty data because of different node voltage reference

To prevent noise over long distance cable, we can earth one side of the
cable shield (refer to Ott, Henry, Noise Reduction Techiques in
Electronic Systems).

To prevent faulty data because of different node voltage, we can use
common line.  But since RS485 is a differential mode protocol, we can
use either A or B line as our common line.

However, if we connect using this fashion, when the master is in the
idle mode, there will be floating voltage between A or B line (since
nobody is driving the bus). Therefore, we connect pull-up/pull-down
resistors in the A and B line (to give at least definite voltage level
when nobody's driving the bus).

To make this "definite voltage level" same at the receiver /
transceiver point, we need to earth their voltage reference node at the
both side.

I think everyone is correct here. Just the naming convention that makes
confusion.

Peace everyone =p

-kunil


Re: RS485 is bidirectional does it mean it is fullduplex?
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Yes.


I don't see how you can use A or B as a "common" line.  

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What happens when nobody is driving the bus is a (mostly)
different issue. Usually solved by pulling one line to the
reference/common node and the other to 5V (with respect to the
reference node).

The reference node connection between the two ends is required
to keep the A/B signal values being output by the transmitter
within the common-mode voltage range spec for the receiver.

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You don't need to earth either one, as long as the
transmitter/receiver reference nodes at the two ends are tied
together.

--
Grant Edwards                   grante             Yow!  Bo Derek ruined
                                  at               my life!
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Re: RS485 is bidirectional does it mean it is fullduplex?
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On a "long distance cable", you'd better ground *both* ends.

First, understand that a "shield" has virtually no effect at 60
Hz.  The reduction in induced signal at 1000 Hz is about 3 dB
when a shield is added.  At 60 Hz the difference is about 0.04
dB!  (Which says, we don't put a shield on the cable to
necessarily reduce noise in the obvious way!  It has other
effects, if used correctly.)

A "ground loop" is caused by having a *common* ground path for
two signals.  Hence if the "ground" for a cable shield is
provided by attaching it to the equipment, and most particularly
if it is attached in a way such that from the connection to some
other point there is a shared path with the signal, current in
the cable shield will affect the signal to the degree that it
can cause a voltage drop across the distance of that common
connection.  That can be significant at higher impedances.

That is the *wrong* way to ground a long cable.  I've emphasized
a separate ground cable is required, and that buildings require
a single point ground system, just to avoid said ground loops.

Here it is graphically.  This is an incorrectly grounded cable
shield, causing a ground loop with each equipment.  All currents
induced into the cable shield share the common connection to
ground *through* the equipment.

     +-------+                                    +-------+
     |       | >--------- tx wire/pair ---------> |       |
     | EQUIP | <--------- rx wire/pair ---------< | EQUIP |
     |       |  ========= cable shield =========  |       |
     +-------+  |                              |  +-------+
         |  |   |                              |   |  |
         |  +---+                              +---+  |
         o                                            o
         |                                            |
       -----  Earth                                 -----  Earth
        ---   Ground                                 ---   Ground
         -                                            -

By the expedient of removing the ground at one end, several things
are accomplished.  One is the removal of the ground loop.  It also,
however, removes common mode DC equalization, and it reduces the
current flow in the cable shield, which happens to have a negative
effect, as I'll show.

Note that this is *very* appropriate for use with cable existing
within a single building.  The benefit is the same, but the
negatives are of negligible effect.


     +-------+                                    +-------+
     |       | >--------- tx wire/pair ---------> |       |
     | EQUIP | <--------- rx wire/pair ---------< | EQUIP |
     |       |  ========= cable shield =========  |       |
     +-------+  |                                 +-------+
         |  |   |                                     |
         |  +---+                                     |
         o                                            o
         |                                            |
       -----  Earth                                 -----  Earth
        ---   Ground                                 ---   Ground
         -                                            -

However, if the cable is a long run, and particularly if there
is exposure to power lines, if the ground potential is different
at the two ends, or if there are any other sources of induced
noise in the cable, this arrangement has the best effect:

     +-------+                                    +-------+
     |       | >--------- tx wire/pair ---------> |       |
     | EQUIP | <--------- rx wire/pair ---------< | EQUIP |
     |       |  ========= cable shield =========  |       |
     +-------+  |                              |  +-------+
         |      |                              |      |
         |      |                              |      |
         o------+                              +------o
         |                                            |
       -----  Earth                                 -----  Earth
        ---   Ground                                 ---   Ground
         -                                            -

Note the minimum common path to ground.  If correctly sized
there will be no significant voltage drop across that small
section.  (Which is to say, that should probably be copper strap
between a copper terminal plate and the actual ground system
connection.)

Hence, there is no "ground loop" effect.  However, the two
grounds are connected electrically and the voltage is equalized
between then.  The second benefit is that voltages induced into
the cable by exposure to electro magnetic fields will have a low
impedance circuit path, and will therefore conduct current.

Just as it does in a transformer, current changes in one
direction cause a opposite voltage to be induced into a coupled
conductor.  Hence, we have the external field causing a voltage
in the shield and the pairs which is identical.  The current
that flow is the shield causes an exactly *opposite* voltage to
be induced into the cable pairs.  The externally induced voltage
and the shield induced voltage cancel to some degree in the
cable pairs, thus reducing external noise induction into signal
pairs.  This effect *requires* both ends be grounded (with
quality connections presenting a relatively low impedance to the
noise current).

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That is indeed a significant part of the problem.

The other part is just not being exposed to the full expanse of
what is involved in data transmission over longer lengths of
twisted pair cables.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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If you really do need to connect at both ends then you may need to consider
inserting some impedance in the screen connections at both ends (usually a
capacitor and resistor in parallel). The resistor is large enough to
prevent high current flows but needs to be small enough to provide an
effective electrostatic drain. The capacitor provides a low impedance at
higher frequencies.

--
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Re: RS485 is bidirectional does it mean it is fullduplex?
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No, the whole idea is that you *want* that current to flow.  In
particular it is the 60 Hz power line induced current that makes
up most of the current flow.

Keep in mind that the whole idea is to allow the current flow to
generate an equal and opposite induction into the signal pairs.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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I don't think anyone wants 53A being carried by the shield of a signal
cable. I know I certainly wouldn't like to see that happen. This is the
sort of thing that we have been trying to get you to see as a real risk for
some of the systems we are dealing with. I have even seen scope leads fry
due to someone not respecting the earthing scheme in place (on a high power
motor drive).
 
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If you are speaking of twisted pair screened cable (the type in very
extensive use in my workplace) then I thought that we had already agreed
that the twist in the cable cancels out most of the noise of a signal
because the noise is a common mode across the pair. The screen, in this
case, really does extend the shielding of the enclosure out to the plant. I
am certain that these principles are in many of the books on
telecommunications and electrical theory.

--
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Re: RS485 is bidirectional does it mean it is fullduplex?
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And they need not worry that it will, either.  What's your point?

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Yes, and you have a *very* unusual environment too.  Are you suggesting
that everyone else engineer their equipment to match something they will
*never* encounter?

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I'm just overjoyed that you are aware of that.  However, in many instances
it is simply *not* enough.  That is why virtually *all* telephone cables
are installed as I've described, with a good earth ground at *every*
point were sections of cable are spliced.  That is commonly either at
3000 or at 6000 feet.

Now, you can cite all the less than authoritative sources on the Internet
that you like, and concoct all the unusual circumstances you'd like too,
the *facts* are not going to change, and the standard practice is exactly
what you say can't be done.

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You do realize that the "shield" effect, at 60 Hz power line
frequencies, reduces noise in a cable by about 0.04 dB?  In
other words, it has no effect at all.  I'd have to look up the
numbers, but it essentially has little effect at any frequency
below about 10KHz.  Obviously the shield on a telephone cable is
*not* there to reduce noise simply by keeping stray
electro-magnetic fields out of the cable.

If you want a good book, try "Telecommunication System
Engineering", 3rd Edition, 1989, by Roger L. Freeman.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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You will find the sort of environment I speak of is similar to most
factories. I just get it about 3 times as bad because of the really high
energy that we are dealing with in two parts of our site. That really is
not that much of a margin above most of the others. Perhaps a profile of
the group members working environments might show you how many share
conditions similar to those I have dealt with for more than 30 years.

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As I asked in another, are you speaking of armouring or shielding?
 
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At 50Hz, probably 60Hz, that may be the case on a good day. However, I have
other techniques that deal with the 50/60Hz noise issues which do not rely
on the shield. However, we have a number of electronic motor drives, 30MHz
RF sources (3 by 8MW) microwave systems of about 4MW and several cameras
within the experimental zone. The shielding is effective at those
frequencies.

--
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Re: RS485 is bidirectional does it mean it is fullduplex?
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MEGA-Amps... ?

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Are you still confused?

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Yes it is.  But we were talking about 50/60Hz power, not about
30 MHz RF, which had not been mentioned until now.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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... snip ...
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Yet use of the old fashioned 20 ma current loop, driven through
photo diodes/transistors, is virtually immune to these problems.
It is biased in only one place.  If you are going to inject signal
at every station the interface is harder.  For point to point or
broadcast you can hardly beat it.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: RS485 is bidirectional does it mean it is fullduplex?
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Frame ground is "chassis ground".

For example, Pin 1 on the 25 pin RS-232 connector is variously
labeled as "Chassis", "Protective", "Shield", or "Frame" ground.

Pin 7 is "Signal Ground".

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Well, the opposite side of that would be "somehow guarantee use
of receivers that can handle the existing common mode voltage
excursions".  (Note that I am specifically not limiting that to
DC.)

The point is not that there is no DC connection to the outside
world, but that is has to be done *correctly*.  And that is not
accomplished via a single ended one wire loop added to the
required pairs.

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I'm not talking about DC.  I'm talking about how to reduce
*noise* in a communications cable.   Very few such cables
operate in an environment where there is no significant power
line influence, not to mention other noise sources.

If the ground system is properly designed, the noise in the
cable is reduced.  If not done right, it can be substantially
increased.  And it can exceed ground potential difference by
several times, too.  There is no point in reducing the DC
ground potential from 10V to 0V, and in the process acquiring
20 VAC in the process.

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The RS-485 signals are carried on a cable.  Any influence on the
output which is not the input signal, is noise.  It is
impossible to avoid (particularly 60 Hz power influence).  One
reason RS-485 was only specified for 4000 feet is because it
isn't very immune to noise.

DC common mode offset is just another noise...

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It was originally spec'd at 4000 feet.  Better line driver
technology has extended that.

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If he had significant experience with comm cables, which is most
likely, then your cable had a properly grounded shield and had
surge protection installed at both ends.

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The RS-485 at each end is connected to the same ground that
the cable is connected to.  But there should *not* be a
cable pair dedicated to connecting the two.

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Ground that is not a signal path.  E.g., common mode ground.

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It's necessary to grasp the difference in what "ground" is,
and I'm not really aware of what your exposure to it is.  I was
assuming that since you wanted to talk about RS-485 at the
hardware level that you'd probably been exposed to all of this,
but wouldn't necessarily have remembered it or found any of it
significant.  In that case, simply using the vocabulary correctly
will enforce a proper set of definitions on the discussion.

But if you aren't into electricity for the sake of electricity...
yeah, this starts getting to sound like word soup!  Sorry about
that.

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What you are doing will result in equalizing the common mode DC
offset from different grounds.  It is *not* the best way to do
it, simply because it can (not necessarily, but *can*) cause
just as many problems as it solves.  Done properly, you don't
have trouble with 1) common mode offset, 2) induced AC and other
transient, or 3) lightening surges.  But any of those can be
handled in other ways... which increase the potential for trouble
with one of the others.  The shorter the cable run, and the fewer
hazards it is exposed to, the fewer problems.  Hence it can easily
be done in ways that are not the best, and yet work very well for
years.  But that doesn't mean those methods are "correct".

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They should be.  But you've got two different circuits you are
talking about too.  One on each side of the optical isolation.
On one side the common mode range is narrow, and on the other is
is very high.  The isolators are used over the cable, so ground
potential offset is not a problem (because the offset voltage
will never approach the common mode limit for the optical
isolators).  On the other side, they are *all* connected to a
common ground, if they are properly engineered.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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Except that none of the RS-485 circuits were grounded.  They
were all optically isolated and the RS-485 transmitter and
receiver grounds were floating.

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They aren't.  The product spec required that the RS-485 bus be
isolated. It's pretty common (at least in the type of gear I
saw) for RS-485 interfaces to be optically isolated.  The
RS-485 transmitters and receivers had floating grounds.

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No, the opto-isolators were between the RS-485
transmitters/receivers and the rest of the gear.  The Rs-485
transmitters and receives were galvanically isolated from
earth, chassis, and supply ground.  Our experience was that
connecting two "floating" RS-485 interfaces together without a
common (connecting just the data lines) resulted in a lot of
problems.

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--
Grant Edwards                   grante             Yow!  I'm having an
                                  at               emotional outburst!!
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Re: RS485 is bidirectional does it mean it is fullduplex?
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I'm confused about what you actually have, see below.

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I wasn't getting a clear picture of this yet... and I'm not
sure that I am yet! :-)


 EQUIP  ISOLATOR  RS485 <===/ CABLE /===> RS485  ISOLATOR EQUIP

If you are doing that, with 10 km of cable and *not* grounding
the cable shield, it may well work, but it isn't the best way to
do it..

You don't need to use a cable pair to connect the common mode
grounds on the RS485 drivers, because the shield should have
a separate ground strap going to a building ground at each end.
The RS485 common mode ground points should also be connected
to those same building grounds, with a separate cable.

 EQUIP  ISOLATOR  RS485 <===/ CABLE /===> RS485  ISOLATOR EQUIP
                     |   |             |   |
                      \ /               \ /
                       |                 |     <--  COMMON
                     -----             -----
                    / / /             / / /

The part marked as COMMON must be sized appropriately to avoid
ohmic losses from current in the cable shield from affecting
the RS485 drivers.  Generally that means either a very large
cable, or a very short distance.

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--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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I don't think I ever saw runs longer than 2-3km.

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The cable shield was grounded.  What wasn't grounded was the
RS-4xx driver/transceivers.

----+     +------+             +------------/  /-- Shield
    |     | Opto |      +-----+ \
  uP|-Tx--| Iso  |--Tx--| 485 |--|---A-----/  /--
    |-Rx--|      |--Rx--| xcvr|  |
    |-dir-|      |-dir--|     |--|---B-----/  /--  Mirror
-++-+     |      |      +-++--+  |                  Image
 |`--Pwr--| Pwr  |-IsoPwr-'|     |
 `---Gnd--| Iso  |-RScom---+-----|---com---/  /--
      |   +------+              /
     ///                       +--+--------/  /--
                                  |
                                 ///

Or something pretty close to that. In some pieces of gear the
uP was floating (uP ground was RS485 common).  In all cases the
RS-485 interface was floating with respect to earth/chassis
ground.

I don't remember if people were told to ground the shield at
one end or both.  There were A, B, common, and chassis ground
lugs on each of our bits of gear (in addition to some other
stuff unrelated to the discussion).

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That's sure not the way I remember it in the process control
systems I worked with (it's been 6-7 years). The RS-485 bus was
pretty much always floating w/ respect to ground, with A, B and
a floating common wire between the two RS485 transceivers.
There may have been people that grounded the RS-485 common node
at some point, but it was expected to work if it was floating.

--
Grant Edwards                   grante             Yow!  Psychoanalysis?? I
                                  at               thought this was a nude
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Re: RS485 is bidirectional does it mean it is fullduplex?
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Okay.  That's just fine.

Hmmmm...  It just occurred to me that if this goes into customer
premise locations, the benefit is obvious.  That is *much*
less complex than telling someone they have to install an
appropriate ground system!

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I suspect that was engineered around less than the best customer
premises.  They can't control the customer's environment, so
designing it to avoid that altogether is a smart thing to do.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?

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I think that was the general idea.

The people who installed this stuff were much more used to
installing 4-20mA current loop instruments, which were always
isolated from ground, and the 4-20mA current loop could be
grounded at any point in the loop (but hopefully only at one
point).

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RS-485 was pretty darned high-tech at the time.  We were using
it to replace a weird proprietary busd that used 48V
differential signalling (with a floating common) that ran at
two different baud rates.  The "high speed" version was 250
baud, IIRC.  A big advantage of RS-485: it doesn't hurt nearly
as much as 250Hz at 48V.  The 48V was current-limited but it
still stung.

--
Grant Edwards                   grante             Yow!  Okay... I'm going
                                  at               home to write the "I HATE
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Re: RS485 is bidirectional does it mean it is fullduplex?
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On the contrary, it has high noise immunity (i.e. the common-mode voltage
range). It's a balanced differential system - any noise induced on one
signal is likely to be induced in equal measure on the other, and hence
cancelled out.

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
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Like I said, low noise immunity.  With a very low common mode
dynamic range, for one thing.

If you want something with better noise immunity, look at the
DS1 interface specifications.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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Eh?

If you mean the +/-7V common-mode range, yeah, it's not huge - but it's more
than adequate if correctly applied.

Steve
http://www.fivetrees.com



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