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Re: RS485 is bidirectional does it mean it is fullduplex?
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Every telephone cable in the country has the shield connected
to ground at both ends.

100 A of AC!  Where are you getting these numbers for 100's of volts
and 100's of Amps?

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There won't be any 10 A of "dirty 50/60 Hz AC".

As I've said *every* telephone cable in the country is grounded
at both ends.  Take a look at every splice box you can find, and
you'll see a ground wire running down the pole to a ground rod.
Each and every section of cable is grounded at both ends.

Whatever electric fields that may cause induction into the cable
will cause significantly more (and no it will *not* be any 10 A)
current in the much lower resistance shield than in the
individual pairs.  Plus the pair will not be grounded (although
grounding spare pairs at both ends will have the same effect as
grounding the shield), or at least not through as low a
resistance as the shield.  Hence sigificantly more current will
be flow in the shield as a result of any induction.  That
current flowing in the shield will produce an *opposing* field,
that will cancel at least some of the noise induced into the
individual pairs!

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And absolutely unnecessary too.  RS-422 works on two pairs.  The
master transmitter and all slave receivers are on one, and the
master receiver and all slave transmitters are on the other.
They are *all* high impedance devices, relatively.  There has to
be a 100 Ohm load resistor on each pair.

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Unnecessary for RS-422.

--
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 can assure you that the screen of my telephone cable is NOT connected to
ground on my side.
Which is logical too, because you want to keep any current through the
screen as low as possible and that can onlybe achieved by connecting the
screen at only one side.

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And all those slave transmitters should be disabled, except the one that's
talking to the master.

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So, if all slaves have a dominant Mark, how is one slave going to drive a
Space on the line?

Meindert



Re: RS485 is bidirectional does it mean it is fullduplex?
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Then I assure you that 1) you are either talking about a drop
cable, which does not have a sheild, or 2) who ever installed it
was incompetent.

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Wrong.  You want to have any current induced into each
individual pair to be *more* significant in the shield than in
the pair.  The shield, being grounded at both ends, will have
current flow that will set up an *opposing* field to the
original source, and since it is fairly closely coupled to the
cable pairs, it will in fact cause at least some cancellation of
noise on the pairs.

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They are high impedance devices.   The only "disable" they need, is
to not be sending.

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If all of them try to send at one time, you get garble.

--
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?
On Thu, 16 Jun 2005 10:27:48 -0800, snipped-for-privacy@barrow.com (Floyd L.

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You seem to confuse the RS-422/485 with CAN bus.

In CANbus, the transmitter is only actively sending the dominant state
("0"), while passive pull-ups put the bus into the recessive state
("1"). All inactive transmitters are constantly "sending" the
recessive state and only the active transmitter and only when sending
the dominant state is actually sinking/sourcing current to the bus.

Prior to dedicated CANbus transceivers, ordinary RS-485 transceivers
were used, with the transmitter input tied constantly to "0" and the
data stream connected to the transmit enable pin ("0" enabled the
transmitter) to generate the dominant state on the bus.

Similar multidrop dominant/recessive state behaviour can be created
with RS-232 and RS-422 without transmit enable using one or two
diodes.

Paul


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

Well, 1) someone from the telephone company installed it and 2) is does have
a shield. This is just the way it is done in the Netherlands.

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How can that be? An induced current in the shield will cause, given high
enough frequency and long enough cable, induce almost the same current in
the pair, as common mode. Just look at the principle of a transmission line
transformer. The goal is to induce as little current as possible in the
shield. And that can only be achieved by grounding it on one side.

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Yes, but the pair inside the shield cannot "see" the original source, so
nothing is cancelled there.

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They are NOT high impedance devices. Go check a datasheet. For instance, the
MAX485 has an open voltage of 5V, and  2V when loaded with 50 Ohms. That
represents an internal impedance of 75 Ohm.
Not sending is not the same as being disabled. Not sending means to be in an
idle (mark) state, but still driving the line into that state. Disabling
means shutting down the driver to become high impedant.

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Yes, that's why they have a control input to take the transmitter off line.

Meindert



Re: RS485 is bidirectional does it mean it is fullduplex?
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I can assure you it is *not* "just ... done" that way in the
Netherlands any more than it is here.  If for no other reason
than surge suppression, there *is* a ground and the main cable
*does* have a grounded shield.  There is almost *certain* to be
a ground at your interface to the telephone company.  Whether
you can see it or not, I certainly don't know.

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See the other article I posted explaining that in detail.

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The shield has no such magical effects.  At 1000Hz it will
reduce a noise signal by about 3 dB, and 60Hz it has about 0.04
dB effect.  Which is to say that shielding T1 leads has
significance (and indeed, inside a building ABAM shield cable is
used), but where the primary noise induced will be power line
frequencies and harmonics, shielding as such has little effect.

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RS-422 is specified as maximum open voltage +/- 10 V, when
loaded with 100 Ohms, +/- 2V.  Driver output resistance, 100
Ohms, maximum current 150 mA.  Receiver input resistance 4000
Ohms, common mode +/- 7 Volts, and sensitivity +/- 200 mV.

Hence, if there are 10 transmitters on the line, the requirement
is more than 200mV with less than 150mA...  which is a very low
resistance.  Even if the 100 Ohm output resistance represented a
load resistance, that would make a total of 11 such loads across
the line, and the impedance would be just less than 10 Ohms.
That 150mA of current would generate well over a volt of
signal.

Regardless, I'm *not* positive what does or does not work.  I
don't recall ever using RS-422.  In searching the web I find
reliable sources (Maxim, for example) who say that 1 master and
10 slaves, which they diagram as having both transmit and
receive functions, can be used.  On the other hand, I've also
seen very explicit statements that RS-422 can handle 1 master
that can transmit and receive, and *all* of the slaves are
receive only.  Or statements that a "true" multidrop cannot be
done with RS-422.

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That is only *one* way to accomplish that.

--
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've just confirmed my suspicions that you have no clue what you're
talking about. RS-422 and RS-485 drivers are low-impedance in both MARK and
SPACE state, and do indeed have a third (tristate) condition.

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
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Impedance is relative.  To an 8 Ohm speaker,  600 Ohms is high impedance.

--
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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Errr... no, the 100R is there for transmission line matching. It should
match the characteristic impedance of the cable.

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
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So?  You seem to miss the fact that its DC characteristics are
just as important a its impedance matching function.  That's the
same as the 50 Ohm termination on a 10base5 ethernet.  Ever try
using RG59 cable instead of RG58?  It works... as long as you stick
with 50 Ohm terminations.  Same point...

The fact is the 100 Ohm load resistor, being of lower resistance
than any other load on the cable, is what determines the voltage
levels.  The receivers are high impedance voltage sensing devices.

--
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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That's a bit simplistic. The driver signal hits the characteristic Z of the
cable first; *that's* what determines the voltage levels. It then travels
along the cable, and eventually hits the last receiver, and its termination
resistor. Ideally, there is at that instant no change in impedance, and
hence no energy to dissipate/find some other how.

In terms of what the receiver sees, however, I agree. But it's not what
(common-mode) we've been arguing about ;).

(BTW, it's now 3:38am here in southern UK, and probably a bit warmer than
where you are ;). Nice pics on your site.)

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
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The voltage on the cable is determined by that 100 Ohm resistance,
not by the impedance of the cable.

If you don't believe it, go find a pair of old 10base5 ethernet
NICs and connect them together with *any* coax of your choice.  It
was designed for 50 Ohm coax with a 50 Ohm term on each end.  But
try it with the commonly available 75 Ohm cables used for cable TV.

It will work just fine...  unless you use a 75 Ohm termination.
That will kill it dead.

Lacking the ability or opportunity to do that, try reading up on
in it.  You can find that described in any number of places.

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What the receiver while see, no matter where it is on that
cable, is the voltage across that 100 Ohm resistor.  And yes
that is exactly where common mode rejection takes place.

And it happens anyway, that the impedance of the cable will almost
certainly be something between 120 and 150 Ohms anyway.

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Thank you.  It hasn't been updated for awhile.  I do have
another photo essay that I should finish off and make available.
It's on skin "umiaq" boats.  (There are also other parts of the
web page that aren't accessable via the homepage, where I have
some programing available.)


--
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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The first clue is in the RS-422 spec; I quote:
"The driver has the capability to (...) drive up to 10 parallel connected
receivers."

The second clue is in the fact that many of the old RS-422 drivers had
tristate control inputs. (I would agree, however, that this appears not to
be mentioned in the RS-422 spec.)

In any case, multidrop RS-422 was widely used. (All the comms for all the
products from the company I was working for in the late 70s and 80s were
done this way.) It worked.

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
On Thu, 16 Jun 2005 13:57:44 +0100, "Steve at fivetrees"

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My point was that it is not mentioned in the standard.

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A question of semantics, should such system be called a multidrop
RS-422 system or a 4-wire RS-485 system :-).

While this configuration is widely used, claiming that it conforms
either to the RS-422 or RS-485 standard would be a bit suspicious.

Paul


Re: RS485 is bidirectional does it mean it is fullduplex?
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Point accepted.


Heh - this was from long before RS-485 was published ;).

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Agreed. Although we were using standard drivers, with tristate control
inputs, from reputable companies (e.g. TI), and not kludging anything - just
completely following and using the datasheet. Also, we could kinda see
RS-485 coming...

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
If you use a 4 wire connection, that is TxA to RxB in one direction (Wher
A is one unit and B  is another) and TxB to RxA in the other, then you ca
work in full duplex. Of course B can represent a number of "outstations
(32 by the original RS485 spec) and provided only one drives the bus a
any given time you can still have full duplex operation.

RS485 allows you to work with a 2 wire connection between A and B. The T
and RX lines are paralleled. Only one driver can drive the lines at an
given time and hence it can only work in half duplex. As far as I know th
RS485 spec only addresses the electrical specification and not any form o
protocol that deals with half or full duplex operation.

Incidentally some manufacturers produce devices that allow up to 12
"outstations" by reducing the load of the receivers relative to th
standard load of the sepcification.
  
I once wrote an in-house app-note on the subject which you can find here
http://www.weidmuller.ca/downloads/pdfs/ca_applNotes/SIP02_991635_RS-485_Primer.pdf

If you want more detail, check out the references you will find th
subject covered in much greater depth.


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


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Half duplex means that only one device at a time on the network
can transmit.

Full duplex is when both (all) devices can transmit simultaneously.

Depending on what RS-485 interface you are using you can have
half duplex or full duplex RS-485.

For half duplex you have a single twisted pair that is connected
to the transmitter and to the receiver of each device. The
transmitters of the devices that are not transmitting must be in
high impedance.

For full duplex you have two twisted pairs. The first is connected
to the transmitter of the master and to the receivers of the
slaves. The second is connected to the receiver of the master
and to the transmitters of the slaves.

But since you can only have one device transmitting at a given
time on the same twisted pair, even in full duplex RS-485, the
master must initiate the transfer by selecting a slave. Then both
the master and the slave can transmit in full duplex (simultaneously).

This is purely a problem of protocol, i.e. software.
RS-485 full duplex interface chips allow two devices to transmit
simultaneously. You must arbitrate when you have more than
one slave.


Re: RS485 is bidirectional does it mean it is fullduplex?
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The 485 spec defines an electrical bus consisting of 2 wires and thats
it. It talks about what the loads of each transceiver must be and the
amount of ground difference the bus can take without causing an "off"
transceiver to turn on unexpectantly. The fun thing was that they never
tried to specify connectors or anything handly like that, so the spec
was widely excepted and generally different from one machine to the next
(though screw terminals, I suppose became the defacto standard).

The 4-wire 485 is two 485 buses where one is turned into a
unidirectional transmitter bus from master to slaves. The master is
generally always transmitting and the slave always receives. The other
is receiver bus and is multidrop in the 485 sense (except the master
usually doesn't talk in this scenario). This is a scenario I actually
saw in the RS-422 spec however true RS-422 didn't have the same
electrical spec as 485 (couldn't handle as many drops and had a
different differential gound reference I believe, it has been a while
since I looked at the spec).
The benefit here is that the master should always be able to talk to the
slaves, which can be handy if the return bus has a unit that won't be quiet.

Though the "RTS" has become the signal line of choice for controling
this, I think your instructor is doing you a disservice by specifying it
as some kind of 485 signal. It is never mentioned in the spec, just
something that was handy when they started to marry these things to
Serial controller chips.
A lot of 485 is connected to embedded processors, that generally don't
have RTS lines, it is just another digital io pin.

Tony

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