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'[EE] Duplex & How to connect twisted pair wire tog'
2010\07\05@234045 by solarwind

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Hi all,

I'm creating an RS485 network consisting of a few devices, used for
temperature sensing and control. I want this to be RS485 because it's
simple and I already have the parts/protocols mostly in place. As of
now, it does not need to be multi master, however, I want to leave the
option open.

1. Should I use a half duplex or full duplex RS485 transceiver? I'm
not really sure how full duplex RS485 would work for multiple nodes (4
or 5 nodes) and what the advantages would be.

2. Should I use twisted pair telephone cable or Ethernet cable? Or any
other suggestions?

3. How should all of the "lines" be connected? How about a "junction
box" which consists of a PCB with a bunch of sockets all connected
together? It's simple and clean enough, but I don't know if the
untwisted traces on the PCB will cause any trouble.

2010\07\06@024953 by Ruben Jönsson

flavicon
face
> Hi all,
>
> I'm creating an RS485 network consisting of a few devices, used for
> temperature sensing and control. I want this to be RS485 because it's
> simple and I already have the parts/protocols mostly in place. As of
> now, it does not need to be multi master, however, I want to leave the
> option open.

You also need to specify the maximum speed.

>
> 1. Should I use a half duplex or full duplex RS485 transceiver? I'm
> not really sure how full duplex RS485 would work for multiple nodes (4
> or 5 nodes) and what the advantages would be.

Half duplex is two wires + ground and possibly shield. Both transmitter and
receiver on all nodes uses the same wires for communication. Only one can talk
at a time. Half duplex is the most common for RS485 networks.

Full duplex is 4 wires + ground and possibly shield. I have only seen this used
in systems where there is a single master and multiple slaves. From the masters
point of view it has two dedicated wires for tx and two for rx. This means that
the master can receive and transmitt at the same time. The tx on the master is
connected to the rx on all slaves and the rx on the master is connected to tx
on all slaves. This has the advantage that the master can keep the tx driver
enabled all the time and if the master is a PC this can make the software much
easier to write since there is no built in mechanism in a PC to handle
enabling/disabling of the tx drivers (the software works just like with RS232).
The turnaround of the tx lines on the slave nodes still have to be handled
though.

>
> 2. Should I use twisted pair telephone cable or Ethernet cable? Or any
> other suggestions?

You should use twisted pairs but you also need a third ground wire which
connects the ground(plane)s between all your nodes. This can become a problem
if the nodes are not floating and/or there is a voltage difference between the
grounds on the nodes in the network. A lot of people will tell you that you
only need two wires for RS485 and many times this will work. However, a proper
RS485 network needs a common ground wire - it is not a current loop. There is
more on this in the linked articles below. If there is a problem with different
ground levels in your nodes you can use isolated RS485 transcievers. If you can
build your nodes with an isolated power supply (mains transformer) or you can
supply all nodes from the same power supply and the nodes are not directly
connected to other devices you should be fine.

If you use a shield, it should be connected at both ends to a solid ground
(earth) which is usually the metal frame around a system (chassis). This may or
may not be the same as the ground wire (which is connected to the ground plane
on the boards). This can also mean that you will get problems with ground
currents (ground loops) where the ground (earth) potential is at different
levels on different nodes. This can be prevented by DC decoupling the shield
with a capacitor (but it is rarely needed if the nodes are within a relatively
close area) and it is also usually a lot harder to get a good EMC result with
this setup.

You can use a shielded or unshielded cable with two twisted pairs where one
pair is for communication in the half duplex network and the other pair is for
power.

>
> 3. How should all of the "lines" be connected? How about a "junction
> box" which consists of a PCB with a bunch of sockets all connected
> together? It's simple and clean enough, but I don't know if the
> untwisted traces on the PCB will cause any trouble.

You should connect all the nodes in series with short stubs (bus topology) and
not as a star network. When I say "in series" I mean on a long bus without
stubs or trunks - although the nodes are theoretically connected in parallell.
I usually have terminals for an incomming and an outgoing bus line on each node
that are actually connected in parallell on the board. If you have lots of
nodes the (very low) series resistance introduced in the bus by the terminals
could become a problem. Then you could just use one terminal and connect both
the incomming and outgoing wires together in that terminal. This is mostly a
problem if you supply all boards with power on the bus wires and each node
consumes relatively high power. The low series resistance is not a problem for
the communication wires.

The tx pair should be terminated at the far ends (only one place) with a
termination resistor that matches the cable (usually around 120 ohms). If you
use half duplex there is only one pair and it should be terminated at both
ends.

At one place on the bus (usually on the master in a single master system or at
one end on the bus) there should also be some weak pullup and down resistors to
guarantee that the bus is read as idle when all transmitter drivers are in the
off state. I usually have two jumpers on the node boards which are both used
together to connect the termination resistor and also the pull up/down
resistors.

The RS485 transcievers are pretty tolerant for common mode voltages (-7V to
+12V) but this is not always enough. I always put some low value resistors and
three high powered double (antiparallell) tvs's on each node (two between the
communication lines and ground and a third between the lines themselves).

For some good, solid information about RS485 networks see the following links:

<http://www.national.com/an/AN/AN-1057.pdf>

<www.circuitcellar.com/library/ccofeature/perrin0799/c79bppdf.pdf>

/Ruben

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
spam_OUTrubenTakeThisOuTspampp.sbbs.se
==============================

2010\07\07@232244 by solarwind

picon face
2010/7/6 Ruben Jönsson <.....rubenKILLspamspam@spam@pp.sbbs.se>:
{Quote hidden}

Thanks a lot for the detailed reply. I took a look at the links you gave me.

The way that the network nodes will be placed will make a star
topology the most convenient choice. What's the disadvantage of a star
topology?

Also, if there are only two devices, and a direct RS485 connection
between them (distances anywhere from 1 m to 1000 m) over a twisted
pair line, does the line need anything special? That is, does it need
to be terminated? If so, how do you terminate a point to point line?

2010\07\07@232848 by solarwind

picon face
On Wed, Jul 7, 2010 at 11:22 PM, solarwind <x.solarwind.xspamKILLspamgmail.com> wrote:
{Quote hidden}

Also, if I'm going to daisy chain the devices, how do I terminate the line?

Another idea is to make a very very simple RS485 hub using (almost
nothing but) MAX485 type chips. One can connect the the output of each
chip to the input on all the other chips. This way, anything
broadcasted by one node will be sent to every other node. However,
what are the issues with this. Also, how does one terminate the line
in this case?

2010\07\07@233602 by Harold Hallikainen

face
flavicon
face

>
> The way that the network nodes will be placed will make a star
> topology the most convenient choice. What's the disadvantage of a star
> topology?
>
> Also, if there are only two devices, and a direct RS485 connection
> between them (distances anywhere from 1 m to 1000 m) over a twisted
> pair line, does the line need anything special? That is, does it need
> to be terminated? If so, how do you terminate a point to point line?
>

A 485 bus needs to be a linear bus with terminations at the ends. You can
then drive the bus at any point with a zero ohm driver. The signal goes
down the transmission line in both directions. If it were not properly
terminated with a termination equal to the characteristic impedance of the
line, you'll get reflections which can mess up your data.

If you use CAT5 cable, you can make it look like a star but wire it as a
bus since the cable has multiple pairs. The signal goes out one pair, hits
your equipment, then goes back down the other pair. So, the bus zips back
and forth between the center and each transceiver. There's a termination,
as discussed above, at each end of the bus.

As to the line being "special," it's a transmission line, so it should
have a constant characteristic impedance. This is where Ethernet cable is
great.

Good luck!

Harold




--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2010\07\08@003006 by Bob Axtell

face picon face
Harold Hallikainen wrote:
{Quote hidden}

In the casino days, I used CAT3 (telco two-pair twisted cable). Only ONE
pair was guaranteed to be OK, so I
used a system where the Bus master  gave the command, then  released the
bus to hear the reply. It worked pretty well.
The reason it was used was that the building was very old and that old
cable was all we had to work with.

PS, for you who asked, I survived my "starter heart attack" and have
recovered much of my strength through an
imtense physical therapy program. But I have mostly retired now, and
plan to see a few sights out here in the west, like
Custer's last stand, Meteor crater, Monument Valley, etc. They are all
on my "Bucket List".

--Bob A

2010\07\08@040252 by Michael Watterson

face picon face
 On 08/07/2010 04:28, solarwind wrote:
> Also, if I'm going to daisy chain the devices, how do I terminate the line?
>
> Another idea is to make a very very simple RS485 hub using (almost
> nothing but) MAX485 type chips. One can connect the the output of each
> chip to the input on all the other chips. This way, anything
> broadcasted by one node will be sent to every other node. However,
> what are the issues with this. Also, how does one terminate the line
> in this case?
>
You terminate the two ends.

"Listen" with about x10 impedance
"Talk" with a low impedance

That's roughly how coax ethernet works. 50 Ohm only at each end.

2010\07\08@070238 by RussellMc

face picon face
> The way that the network nodes will be placed will make a star
> topology the most convenient choice. What's the disadvantage of a star
> topology?

People have commented on  the "looks like star, wired like bus" approach.

Note that for an electrically true-star, deep magic can be involved as
cable lengths approach a significant fraction of a wavelength. While
this can be dealt with with sufficient understanding and proper
matching, it's usually a game you want to avoid if not necessary to
play.

Long long ago in another lifetime I used a relatively short coax based
ethernet network (under 100 metres, 10 to 15  PCs etc ) that was
formed by joining multiple lengths of BNC terminated coax together.
Length could be added or stubs added by anyone anywhere on the
"network". The network could be persuaded to misbehave or crash
utterly by inserting a piece of the wrong impedance but essentially
visually identical coax almost anywhere on the network - either in the
"backbone" or as a stub feeding a PC or printer.  Much was learned
about what you can't do successfully :-).

         R

2010\07\09@040131 by solarwind

picon face
Another idea is to make a very very simple RS485 hub using (almost
nothing but) MAX485 type chips. One can connect the the output of each
chip to the input on all the other chips. This way, anything
broadcasted by one node will be sent to every other node. However,
what are the issues with this. Also, how does one terminate the line
in this case?

2010\07\09@042314 by Wouter van Ooijen

face picon face
solarwind wrote:
> Another idea is to make a very very simple RS485 hub using (almost
> nothing but) MAX485 type chips. One can connect the the output of each
> chip to the input on all the other chips. This way, anything
> broadcasted by one node will be sent to every other node. However,
> what are the issues with this. Also, how does one terminate the line
> in this case?

An open line does not need termination in this case (no listener, so why
bother).

But the question is: how do you switch direction?

Or for unidirectional: how do you combine all received data?

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2010\07\09@072516 by Isaac Marino Bavaresco
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face
Em 9/7/2010 05:01, solarwind escreveu:
> Another idea is to make a very very simple RS485 hub using (almost
> nothing but) MAX485 type chips. One can connect the the output of each
> chip to the input on all the other chips. This way, anything
> broadcasted by one node will be sent to every other node. However,
> what are the issues with this. Also, how does one terminate the line
> in this case?
>


If you tie the input of every IC to the outputs of every other ICs, and
vice-versa, you will end with all the inputs and outputs tied together
and you have a half-duplex system.

If the device is a hub, far away from the boards, you will need an extra
wire from each board to switch between transmitting and receiving.


Isaac

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