RS485 is bidirectional does it mean it is fullduplex?
Swizi
RS485 four wire mode has got four pin Rx+, Rx-, Tx+, Tx-. Where the master
device Tx+ and Tx- will be connected to the slaves Rx+ and Rx-
respectively. Simlarly Master's Rx+ & Rx- will be connected to the slave's
Tx+, Tx- . Other than the master all the device will be in receive mode, so
if only there is the transmission RTS is pulled 1 and they(slaves) can
transmitt.
The master is always in receive mode and can also transmitt. In this case
the master is in the fullduplex mode, it is violating the RS485
specification! So could anyone guide me to the right direction regarding
what I have understood.
This message was sent using the comp.arch.embedded web interface on
www.EmbeddedRelated.com
Tim Mitchell
<sw...@rediffmail.com> writes
>I was told by my professor that specification of RS485 is a half
>duplex.
>RS485 four wire mode has got four pin Rx+, Rx-, Tx+, Tx-. Where the
>master
>device Tx+ and Tx- will be connected to the slaves Rx+ and Rx-
>respectively. Simlarly Master's Rx+ & Rx- will be connected to the
>slave's
>Tx+, Tx- . Other than the master all the device will be in receive
>mode, so
>if only there is the transmission RTS is pulled 1 and they(slaves) can
>transmitt.
>
>The master is always in receive mode and can also transmitt. In this
>case
>the master is in the fullduplex mode, it is violating the RS485
>specification! So could anyone guide me to the right direction
>regarding
>what I have understood.
>
RS485 is a 2-wire half duplex system where there is one transmitter and
multiple receivers.
RS422 is a 4-wire duplex system which goes between two devices, each of
which has a transmitter and receiver.
--
Tim Mitchell
antedeluvian
A is one unit and B is another) and TxB to RxA in the other, then you can
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 at
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 Tx
and RX lines are paralleled. Only one driver can drive the lines at any
given time and hence it can only work in half duplex. As far as I know the
RS485 spec only addresses the electrical specification and not any form of
protocol that deals with half or full duplex operation.
Incidentally some manufacturers produce devices that allow up to 128
"outstations" by reducing the load of the receivers relative to the
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 the
subject covered in much greater depth.
>I was told by my professor that specification of RS485 is a half duplex
>RS485 four wire mode has got four pin Rx+, Rx-, Tx+, Tx-. Where the
maste
>device Tx+ and Tx- will be connected to the slaves Rx+ and Rx
>respectively. Simlarly Master's Rx+ & Rx- will be connected to the
slave'
>Tx+, Tx- . Other than the master all the device will be in receive mode,
s
>if only there is the transmission RTS is pulled 1 and they(slaves) ca
>transmitt.
>
>The master is always in receive mode and can also transmitt. In this cas
>the master is in the fullduplex mode, it is violating the RS48
>specification! So could anyone guide me to the right direction regardin
>what I have understood.
>
>
>
>This message was sent using the comp.arch.embedded web interface o
Lanarcam
Swizi wrote:
> I was told by my professor that specification of RS485 is a half duplex.
> RS485 four wire mode has got four pin Rx+, Rx-, Tx+, Tx-. Where the master
> device Tx+ and Tx- will be connected to the slaves Rx+ and Rx-
> respectively. Simlarly Master's Rx+ & Rx- will be connected to the slave's
> Tx+, Tx- . Other than the master all the device will be in receive mode, so
> if only there is the transmission RTS is pulled 1 and they(slaves) can
> transmitt.
>
> The master is always in receive mode and can also transmitt. In this case
> the master is in the fullduplex mode, it is violating the RS485
> specification! So could anyone guide me to the right direction regarding
> what I have understood.
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.
T Marchini
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
Steve at fivetrees
news:uAXCZAWe...@tega.co.uk...
> RS485 is a 2-wire half duplex system where there is one transmitter and
> multiple receivers.
>
> RS422 is a 4-wire duplex system which goes between two devices, each of
> which has a transmitter and receiver.
While this is essentially correct, I'd add:
- Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
actually 5-wire.
- RS-422 is also multidrop (10 max drops, IIRC).
- RS-422 is often implemented using a pair of RS-485 devices these days,
since the RS-485 spec is superior to the original RS-422 spec. This confuses
things slightly, but means that RS-485 tends to be used in both 3-wire and
5-wire configurations.
Finally (pedant mode on), the old RS-422/485 appellation is obsolete:
strictly speaking it's now EIA-422 and EIA-485. (This may be useful to know
for Googling purposes.)
HTH,
Floyd L. Davidson
>"Tim Mitchell" <ti...@sabretechnology.co.uk> wrote:
>> RS485 is a 2-wire half duplex system where there is one transmitter and
>> multiple receivers.
>>
>> RS422 is a 4-wire duplex system which goes between two devices, each of
>> which has a transmitter and receiver.
>
>While this is essentially correct, I'd add:
> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>actually 5-wire.
RS-485 is a differential (balanced) system, and there is no
signal ground connection. The cable used might well include a
frame ground, but that is for noise induction cancellation, not
signal ground.
Hence it actually is a 2-wire or 4-wire link.
Otherwise these are excellent points.
> - RS-422 is also multidrop (10 max drops, IIRC).
> - RS-422 is often implemented using a pair of RS-485 devices these days,
>since the RS-485 spec is superior to the original RS-422 spec. This confuses
>things slightly, but means that RS-485 tends to be used in both 3-wire and
>5-wire configurations.
>
>Finally (pedant mode on), the old RS-422/485 appellation is obsolete:
>strictly speaking it's now EIA-422 and EIA-485. (This may be useful to know
>for Googling purposes.)
Trivia: Even more insignificant (since nobody uses it) the technically correct
appellation apparently is EIA/TIA-485... but I've also seen TIA/EIA-485
and EIA/RS-485 used.
These are pretty good:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/723
http://www.maxim-ic.com/appnotes.cfm/appnote_number/736
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) fl...@barrow.com
Paul Keinanen
Davidson) wrote:
>>While this is essentially correct, I'd add:
>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>>actually 5-wire.
>
>RS-485 is a differential (balanced) system, and there is no
>signal ground connection. The cable used might well include a
>frame ground, but that is for noise induction cancellation, not
>signal ground.
>
>Hence it actually is a 2-wire or 4-wire link.
The practical problem is that the receiver is made of bipolar
transistors and hence require some kind of base current.
This current can be supplied through a more or (usually) less clean
common ground connection or the differential signal path can be
treated as bipolar current loop with the "fail safe" termination
resistors supplying the small base current to the transistors.
Paul
Paul Keinanen
<st...@NOSPAMTAfivetrees.com> wrote:
>"Tim Mitchell" <ti...@sabretechnology.co.uk> wrote in message
>news:uAXCZAWe...@tega.co.uk...
>> RS485 is a 2-wire half duplex system where there is one transmitter and
>> multiple receivers.
>>
>> RS422 is a 4-wire duplex system which goes between two devices, each of
>> which has a transmitter and receiver.
>
>While this is essentially correct, I'd add:
> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>actually 5-wire.
This is generally true in practice.
However, if you are using floating devices and the "fail safe"
termination, the system will work with 2 resp. 4 wires, since the
"fail safe" resistors will force the receiver power supply Vcc and Gnd
potential close to the line potential and thus, within the common mode
range of the receiver (-5..+12 V). Treat the terminated system as a
bidirectional current loop (through the terminating resistors) and it
should be easy to analyze how this works.
> - RS-422 is also multidrop (10 max drops, IIRC).
The RS-422 impedance levels allows for multiple receivers, but I do
not see how a multidrop bidirectional system could be implemented
within the RS-422 specification.
> - RS-422 is often implemented using a pair of RS-485 devices these days,
>since the RS-485 spec is superior to the original RS-422 spec. This confuses
>things slightly, but means that RS-485 tends to be used in both 3-wire and
>5-wire configurations.
The 4 wire RS-485 system is a non-standard strange thing, in which the
master is (nearly) a standard RS-422 device, while all slaves are
eavesdropping the RS-422 downlink but the uplink is a tri-state RS-485
configuration.
>Finally (pedant mode on), the old RS-422/485 appellation is obsolete:
>strictly speaking it's now EIA-422 and EIA-485. (This may be useful to know
>for Googling purposes.)
Good point.
Paul
Floyd L. Davidson
The specification includes the maximum ground offset voltage
permissable. (I don't recall what it is, or how realistic it is
for common 4000 foot runs of twisted pair cable.)
But attempting to supply a separate "signal ground", for example
via a single separate cable pair, will almost certainly result
in a poor frame ground connection instead! That is not a good
idea over a 4000 foot loop, but it probably wouldn't make any
difference at all if the cable run is relatively short. (Unless
it is between two locations on separate power distributions, and
one of them has a bad ground.)
Telephone cable, which I assume is usually what longer runs of
RS-485 would be on, is typically grounded every 3000 or 6000
feet (whatever the spool length is), or less if the cable run is
shorter. That is the outer sheath of the cable, not an
individual pair. (It isn't done often, but grounding all unused
pairs at both ends will also reduce noise induction in the
cable.)
Floyd L. Davidson
>On Wed, 15 Jun 2005 16:17:56 +0100, "Steve at fivetrees"
><st...@NOSPAMTAfivetrees.com> wrote:
>
>>"Tim Mitchell" <ti...@sabretechnology.co.uk> wrote in message
>>news:uAXCZAWe...@tega.co.uk...
>>> RS485 is a 2-wire half duplex system where there is one transmitter and
>>> multiple receivers.
>>>
>>> RS422 is a 4-wire duplex system which goes between two devices, each of
>>> which has a transmitter and receiver.
>>
>>While this is essentially correct, I'd add:
>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>>actually 5-wire.
>
>This is generally true in practice.
It *can't* be true in practice. The presumption is to provide a
signal ground, and it simply doesn't.
Any attempt at providing a signal ground is merely going to connect
frame ground between the two locations, which will very likely cause
more noise induction into the signal pairs than anything else.
>However, if you are using floating devices and the "fail safe"
>termination, the system will work with 2 resp. 4 wires, since the
>"fail safe" resistors will force the receiver power supply Vcc and Gnd
>potential close to the line potential and thus, within the common mode
>range of the receiver (-5..+12 V). Treat the terminated system as a
>bidirectional current loop (through the terminating resistors) and it
>should be easy to analyze how this works.
The circuit impedance is 100 Ohms. It *is* a current loop... :-)
>> - RS-422 is also multidrop (10 max drops, IIRC).
>
>The RS-422 impedance levels allows for multiple receivers, but I do
>not see how a multidrop bidirectional system could be implemented
>within the RS-422 specification.
What would prevent it? It's a fairly simple 4-wire arrangement
with a single master and 10 slaves plus a single 100 Ohm
termination on each cable.
Grant Edwards
> RS-485 is a differential (balanced) system, and there is no
> signal ground connection. The cable used might well include a
> frame ground, but that is for noise induction cancellation, not
> signal ground.
In my experience, the third/fifth wire is required to limit the
common mode voltage seen by the receivers. In that respect, it
is a signal ground. IIRC, most receivers them can only tolerate
8-12V common-mode DC. If you let the two devices float with
respect to each other, you can get fairly high common-mode
voltages and the recievers will stop working.
--
Grant Edwards grante Yow! I know how to get the
at hostesses released! Give
visi.com them their own television
series!
Floyd L. Davidson
>On 2005-06-15, Floyd L. Davidson <fl...@barrow.com> wrote:
>
>> RS-485 is a differential (balanced) system, and there is no
>> signal ground connection. The cable used might well include a
>> frame ground, but that is for noise induction cancellation, not
>> signal ground.
>
>In my experience, the third/fifth wire is required to limit the
>common mode voltage seen by the receivers. In that respect, it
>is a signal ground. IIRC, most receivers them can only tolerate
>8-12V common-mode DC. If you let the two devices float with
>respect to each other, you can get fairly high common-mode
>voltages and the recievers will stop working.
That is a frame ground, and not a signal ground. It will carry
no signal current at all.
And any variation of current seen will be strictly noise. The
trick is to get the induction into the ground wire to then, in
the cable between the ground wire and the signal pairs, cancel
the induction into the signal cables.
What kind of distances have you tried that with? I'd expect
that across the room or around the bend might be just fine (and
wouldn't be needed because the offset between the ground systems
wouldn't be high enough to be a problem). But if this went down
the road 3000-4000 feet, and you actually did get a ground
offset high enough to be a problem, using a single wire in the
same cable to equalize the ground potential should add enough
noise to your cable run to make it a real problem.
A proper ground on each would be much better. And a cable
sheath that is properly grounded at *both* ends, to the same
single point building ground that the RS-485 equipment is tied
to, would be the preferred way to make sure there wasn't too
much common mode difference.
Floyd L. Davidson
>Thought I'd throw my 2 cents in. I used to design systems like this back in
I'm retired now David, but I spent 34 years making systems like
this work, in a variety of environments that would best be described
as mind boggling.
>the 80's and you should be aware that there is always the potential that you
>could cross power grids.
Not in this post, but in another I said:
"it probably wouldn't make any difference at all
if the cable run is relatively short. (Unless it
is between two locations on separate power
distributions, and one of them has a bad ground.)
Note the kicker at the end of that, about a bad ground. That
can mean a faulty ground system, or it can simply mean that a
"good ground" is simply not available.
>I worked on a project where the same company had 2
>buildings across the street from each other and they were on seperate power
>grids. The grid different was not in volts, but in 10's and 100's of volts.
Lets not get too far from reality here. If it was 100's of
volts somebody has a *very* serious fault in the electrical
system.
On the other hand, up to 10 volts is not rare at all.
>It was something we never expected but it's real, and your ground wire won't
>protect against this.
I'm not sure which "your ground wire" you are referencing. A
single wire in the same sheath as the twisted pairs used for
data, won't "protect" against it for the reasons that I stated.
It will probably add more noise to the data circuits, and do
little else. If the ground systems are really bad it might
actually equalize the offset though.
A well grounded cable sheath, at both ends, almost certainly
will correct the problem. Which is to say, I've never seen it
fail, but have seen instances where it was not as good as we'd
have liked to see. Invariably that has to do with inability to
get a good ground connection. But it *is* good enough for
RS-485, as long as the ground systems for equipment on both ends
are in fact connected to the ground system the cable is attached
to. Multiple grounds won't do, even if they are relatively
good. The cable and the equipment both must be tied separately
to a single building ground.
Typically, telephone equipment cannot be adjusted for more than
about 20 volts of ground difference. That's the range of
adjustment provided on most equipment (typically that would be
something referred to as an emx unit, or as dx signaling).
And of course that is with much slower data and much higher
voltages than RS-482.
David Brown
grids. The grid different was not in volts, but in 10's and 100's of volts.
protect against this.
dbrown
"Floyd L. Davidson" <fl...@barrow.com> wrote in message
news:87u0jzj...@barrow.com...
Grant Edwards
> Grant Edwards <gra...@visi.com> wrote:
>>On 2005-06-15, Floyd L. Davidson <fl...@barrow.com> wrote:
>>
>>> RS-485 is a differential (balanced) system, and there is no
>>> signal ground connection. The cable used might well include a
>>> frame ground, but that is for noise induction cancellation, not
>>> signal ground.
>>
>>In my experience, the third/fifth wire is required to limit the
>>common mode voltage seen by the receivers. In that respect, it
>>is a signal ground. IIRC, most receivers them can only tolerate
>>8-12V common-mode DC. If you let the two devices float with
>>respect to each other, you can get fairly high common-mode
>>voltages and the recievers will stop working.
>
> That is a frame ground, and not a signal ground. It will carry
> no signal current at all.
It doesn't carry any signal current, but it is the ground to
which the receiver's input signal range specs are references.
It's the ground that defines what "0V" is for the signal
inputs. I call that the signal ground.
> And any variation of current seen will be strictly noise.
What current?
> The trick is to get the induction into the ground wire to
> then, in the cable between the ground wire and the signal
> pairs, cancel the induction into the signal cables.
I really don't understand what you're talking about. The
differential receiver inputs can deal with only a few volts of
common mode DC voltage. You have to use a ground that's common
between the transmitters and receivers to make sure that the
common-mode DC voltage seen by the receivers is within spec.
> What kind of distances have you tried that with?
A couple kilometers.
> I'd expect that across the room or around the bend might be
> just fine (and wouldn't be needed because the offset between
> the ground systems wouldn't be high enough to be a problem).
> But if this went down the road 3000-4000 feet, and you
> actually did get a ground offset high enough to be a problem,
> using a single wire in the same cable to equalize the ground
> potential should add enough noise to your cable run to make it
> a real problem.
It didn't seem to.
> A proper ground on each would be much better.
Not allowed for safety reasons. The RS-485 transceivers at
both ends are optically isolated from earth.
> And a cable sheath that is properly grounded at *both* ends,
> to the same single point building ground that the RS-485
> equipment is tied to, would be the preferred way to make sure
> there wasn't too much common mode difference.
Nope. The cable sheild is earth ground at one end or the other
and can't be electrically connected to the RS-485 signal or
"ground" signals.
--
Grant Edwards grante Yow! A shapely CATHOLIC
at SCHOOLGIRL is FIDGETING
visi.com inside my costume...
Paul Keinanen
Davidson) wrote:
>The specification includes the maximum ground offset voltage
>permissable. (I don't recall what it is, or how realistic it is
>for common 4000 foot runs of twisted pair cable.)
It would be unrealistic to assume that the grounding electrodes of two
separate buildings would stay within the -7.. +12 V common mode range
at all times (especially during thunderstorms), so optoisolation
should be used to keep the grounds separate. The real question is, is
the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
a fiber optic cable be used instead.
>But attempting to supply a separate "signal ground", for example
>via a single separate cable pair, will almost certainly result
>in a poor frame ground connection instead!
You do not want a good ground connection at least not at both ends in
this situation. It is quite common to use a 100 ohm resistor between
the signal ground wire and the frame ground (PE) to limit the loop
current. Preferably, at least one end of the connection should be
floating (optoisolated) in which case the signal ground wire goes only
to the C terminal of the floating interface, but there is no
connection between the signal ground and frame ground (PE) at that
end.
Paul
Paul Keinanen
Davidson) wrote:
>That is a frame ground, and not a signal ground. It will carry
>no signal current at all.
The ground wire does not carry any signal current. The signal ground C
is required to supply the bias current.
Assume that the receiver input stage consists of a differential pair
made up of NPN transistors. These NPN transistors require that a bias
current flows into the base in order to get a collector current
flowing and thus a meaningful output voltage from the differential
pair.
If you just connect the A wire to one base and the B wire into the
other base, there would be at least one reverse biased junction in the
path between A and B and no bias current would be available and the
receiver would not work.
If there is the ground return, the bias current from either A or B
wire would flow into the NPN transistor base, then through the
constant current resistor and back to C and the stage will now
operate.
The other alternative is that the "fail-safe" termination is used, in
which case a large resistor is connected from receiver Vcc to one base
of the differential stage, the transmission line termination
resistance (typically 100-120 ohms) is connected between the
transistor bases and a large resistor is connected from the other base
to local DC ground. There is a small current flowing through the
voltage divider biasing the differential stage properly.
When the A and B wires are connected to the ends of the terminating
resistor, a large signal current will flow in either direction
depending of the signal being transmitted and hence the voltage
between the transistor bases will also change and the signal can be
recovered. The large bias resistors from Vcc to one input and from the
other input to DC ground will help to keep the floating receiver DC
supply close to the signal pair average potential and hence within the
common mode range.
In this configuration only two wires are required. No signal grounds
wires nor any connection to local frame ground are required, but the
transceiver supply must be floating. The problem with this system
especially in multidrop systems is that the terminating resistors are
at the end of the bus, but the bias resistors must be used at each
station and are effectively in parallel, loading the bus. This may
limit the number of stations connected to the bus.
Paul
Paul Keinanen
Davidson) wrote:
>>>While this is essentially correct, I'd add:
>>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>>>actually 5-wire.
>>
>>This is generally true in practice.
>
>It *can't* be true in practice. The presumption is to provide a
>signal ground, and it simply doesn't.
>
>Any attempt at providing a signal ground is merely going to connect
>frame ground between the two locations,
DO NOT connect the frame grounds together with the data cable
(shield). This cable shield can conduct 1-100 A of AC current, which
should have otherwise gone through the neutral wire.
>which will very likely cause
>more noise induction into the signal pairs than anything else.
10 A of dirty 50/60 Hz AC current with usually a lot of odd harmonics
(especially at 150/180 Hz in three phase systems with electronic
loads) can indeed cause a lot of interface to the data within the
cable. Exactly for this reason, the signal cable shield should NOT be
grounded at both ends.
>>> - RS-422 is also multidrop (10 max drops, IIRC).
>>
>>The RS-422 impedance levels allows for multiple receivers, but I do
>>not see how a multidrop bidirectional system could be implemented
>>within the RS-422 specification.
>
>What would prevent it? It's a fairly simple 4-wire arrangement
>with a single master and 10 slaves plus a single 100 Ohm
>termination on each cable.
IIRC, the original RS-422 specification did not contain specifications
for tri-stating the transmitter, so doing it directly would be a bit
hard. However, both RS-232 and RS-422 can be used in a multidrop
configuration with a few diodes, but this would reduce the noise
margins, since the bus would only be actively driven into the Space
state, while passively pulled by bias resistors to the Mark (which
also is the idle state).
Paul
Floyd L. Davidson
>On Wed, 15 Jun 2005 15:40:22 -0800, fl...@barrow.com (Floyd L.
>Davidson) wrote:
>
>>The specification includes the maximum ground offset voltage
>>permissable. (I don't recall what it is, or how realistic it is
>>for common 4000 foot runs of twisted pair cable.)
>
>It would be unrealistic to assume that the grounding electrodes of two
>separate buildings would stay within the -7.. +12 V common mode range
>at all times (especially during thunderstorms), so optoisolation
>should be used to keep the grounds separate. The real question is, is
>the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
>a fiber optic cable be used instead.
I don't have a lot of experience with thunderstorms, so I can't
really comment on that. Otherwise, that simply is not true.
>>But attempting to supply a separate "signal ground", for example
>>via a single separate cable pair, will almost certainly result
>>in a poor frame ground connection instead!
>
>You do not want a good ground connection at least not at both ends in
>this situation.
You *absolutely do* want a good ground connect at both ends.
>It is quite common to use a 100 ohm resistor between
>the signal ground wire and the frame ground (PE) to limit the loop
>current.
There is no "signal ground wire". What are you talking about?
>Preferably, at least one end of the connection should be
>floating (optoisolated) in which case the signal ground wire goes only
>to the C terminal of the floating interface, but there is no
>connection between the signal ground and frame ground (PE) at that
>end.
I'm sorry, that is just not true, other than optisolation is
not a bad thing.
Floyd L. Davidson
>On Wed, 15 Jun 2005 16:04:56 -0800, fl...@barrow.com (Floyd L.
>Davidson) wrote:
>
>>>>While this is essentially correct, I'd add:
>>>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>>>>actually 5-wire.
>>>
>>>This is generally true in practice.
>>
>>It *can't* be true in practice. The presumption is to provide a
>>signal ground, and it simply doesn't.
>>
>>Any attempt at providing a signal ground is merely going to connect
>>frame ground between the two locations,
>
>DO NOT connect the frame grounds together with the data cable
>(shield). This cable shield can conduct 1-100 A of AC current, which
>should have otherwise gone through the neutral wire.
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?
>>which will very likely cause
>>more noise induction into the signal pairs than anything else.
>
>10 A of dirty 50/60 Hz AC current with usually a lot of odd harmonics
>(especially at 150/180 Hz in three phase systems with electronic
>loads) can indeed cause a lot of interface to the data within the
>cable. Exactly for this reason, the signal cable shield should NOT be
>grounded at both ends.
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!
>>>> - RS-422 is also multidrop (10 max drops, IIRC).
>>>
>>>The RS-422 impedance levels allows for multiple receivers, but I do
>>>not see how a multidrop bidirectional system could be implemented
>>>within the RS-422 specification.
>>
>>What would prevent it? It's a fairly simple 4-wire arrangement
>>with a single master and 10 slaves plus a single 100 Ohm
>>termination on each cable.
>
>IIRC, the original RS-422 specification did not contain specifications
>for tri-stating the transmitter, so doing it directly would be a bit
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.
>hard. However, both RS-232 and RS-422 can be used in a multidrop
>configuration with a few diodes, but this would reduce the noise
>margins, since the bus would only be actively driven into the Space
>state, while passively pulled by bias resistors to the Mark (which
>also is the idle state).
Unnecessary for RS-422.
Floyd L. Davidson
>On 2005-06-16, Floyd L. Davidson <fl...@barrow.com> wrote:
>> Grant Edwards <gra...@visi.com> wrote:
>>>On 2005-06-15, Floyd L. Davidson <fl...@barrow.com> wrote:
>>>
>>>> RS-485 is a differential (balanced) system, and there is no
>>>> signal ground connection. The cable used might well include a
>>>> frame ground, but that is for noise induction cancellation, not
>>>> signal ground.
>>>
>>>In my experience, the third/fifth wire is required to limit the
>>>common mode voltage seen by the receivers. In that respect, it
>>>is a signal ground. IIRC, most receivers them can only tolerate
>>>8-12V common-mode DC. If you let the two devices float with
>>>respect to each other, you can get fairly high common-mode
>>>voltages and the recievers will stop working.
>>
>> That is a frame ground, and not a signal ground. It will carry
>> no signal current at all.
>
>It doesn't carry any signal current, but it is the ground to
>which the receiver's input signal range specs are references.
>It's the ground that defines what "0V" is for the signal
>inputs. I call that the signal ground.
It's a frame ground. A signal ground would be a return path to
ground for the signal. RS-232 has such a signal, and is "single
ended", hence all signal lines share the same common ground
return line.
Differential ciruits are "balanced", the signal is between the
two wires of a pair. Neither of them is at ground, so neither
is called a "signal ground". The signal does not depend on
any relationship to ground.
If course, the receiver typically cannot tolerate a common mode
voltage greater than some specified voltage. That that is not
a signal voltage in any way. It just biases the devices out of
their useful dynamic range.
Optical isolators are are nice because they have a significantly
higher "useful dynamic range". Otherwise, the signal on the
cable is still the same.
>> And any variation of current seen will be strictly noise.
>
>What current?
Induced current into that ground wire accompanying the signal
pairs. That's why you don't want it to be one small wire if
it in the same bundle as the signal pairs.
>> The trick is to get the induction into the ground wire to
>> then, in the cable between the ground wire and the signal
>> pairs, cancel the induction into the signal cables.
>
>I really don't understand what you're talking about. The
It is a bit complex. Many people who work with cables don't
understand it very well. But any one who sits down and studies
it a bit can understand it. It isn't even rocket science! ;-)
>differential receiver inputs can deal with only a few volts of
>common mode DC voltage. You have to use a ground that's common
>between the transmitters and receivers to make sure that the
>common-mode DC voltage seen by the receivers is within spec.
Yes. You also have to be very careful about the currents induced
into said ground connection. Do it the wrong way, and it adds
noise to the signal pairs; do the right way and it will help
cancel noise induced from the same source into those signal pairs.
>> What kind of distances have you tried that with?
>
>A couple kilometers.
That's a pretty good run for RS-485.
>> I'd expect that across the room or around the bend might be
>> just fine (and wouldn't be needed because the offset between
>> the ground systems wouldn't be high enough to be a problem).
>> But if this went down the road 3000-4000 feet, and you
>> actually did get a ground offset high enough to be a problem,
>> using a single wire in the same cable to equalize the ground
>> potential should add enough noise to your cable run to make it
>> a real problem.
>
>It didn't seem to.
What kind of cable was this? Cable you installed, or telco
cable?
>> A proper ground on each would be much better.
>
>Not allowed for safety reasons. The RS-485 transceivers at
>both ends are optically isolated from earth.
The cable still has to be grounded at both ends. That connects
your two frame grounds together too.
>> And a cable sheath that is properly grounded at *both* ends,
>> to the same single point building ground that the RS-485
>> equipment is tied to, would be the preferred way to make sure
>> there wasn't too much common mode difference.
>
>Nope. The cable sheild is earth ground at one end or the other
>and can't be electrically connected to the RS-485 signal or
>"ground" signals.
It *should* be connected to earth ground at both ends. And that
ground point *should* be a single point where *all* frame
grounds for the entire building go.
Typically equipment bays in a single row are strapped together,
though sometimes individual racks will have separate grounds.
There should be a single cable from each row (or each rack if
some racks are isolated) to a common grounding point on each
floor of a building. Each comm cable entrance would be
considered just like an individually isolated rack, and would
have its own ground cable going to the grounding point for that
floor. (What this says is that the cable is *not* connected to
a rack. That a rack in one row is *not* connected to a rack in
a different row. That no two rows share a single cable going to
the ground point. But often racks in one row share a ground
cable, and often equipments mounted in one rack share ground
wires.)
Both ends of the cable should be grounded in that fashion.
Floyd L. Davidson
>On Wed, 15 Jun 2005 18:26:09 -0800, fl...@barrow.com (Floyd L.
>Davidson) wrote:
>
>>That is a frame ground, and not a signal ground. It will carry
>>no signal current at all.
>
>The ground wire does not carry any signal current. The signal ground C
>is required to supply the bias current.
There is no point in repeating what I just said.
>Assume that the receiver input stage consists of a differential pair
>made up of NPN transistors. These NPN transistors require that a bias
>current flows into the base in order to get a collector current
>flowing and thus a meaningful output voltage from the differential
>pair.
Likewise, there is no point in explaining the basics of
differential comparitors.
...
>In this configuration only two wires are required. No signal grounds
Yes. Now that you've arrived at exactly what I said...
>wires nor any connection to local frame ground are required, but the
>transceiver supply must be floating. The problem with this system
>especially in multidrop systems is that the terminating resistors are
>at the end of the bus, but the bias resistors must be used at each
>station and are effectively in parallel, loading the bus. This may
>limit the number of stations connected to the bus.
And this has what to do with the comment you quoted and are
discussing?
Meindert Sprang
news:877jguk...@barrow.com...
> >It would be unrealistic to assume that the grounding electrodes of two
> >separate buildings would stay within the -7.. +12 V common mode range
> >at all times (especially during thunderstorms), so optoisolation
> >should be used to keep the grounds separate. The real question is, is
> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
> >a fiber optic cable be used instead.
>
> I don't have a lot of experience with thunderstorms, so I can't
> really comment on that. Otherwise, that simply is not true.
I have the same experience. I ran an RS-485 network in a factory invonment,
using one twisted pair for data and two other pairs to distribute the power.
Total length appr. 500 meters (1500 feet). Never ever had a problem, all
boards shared the same power supply ground. When a link had to be made
between two buildings, the power was split and only both datalines were
connected. At both ends, the systems we ground. Every thunderstorm blew the
Tx chips.
> You *absolutely do* want a good ground connect at both ends.
The only think you absolutely want is some means to prevent excessive common
mode voltage outside the range of the TX chips. How you achieve this, is not
important. This could easily be done with the "third wire" to connect both
commons (I explicitly won't call it "ground").
Meindert
Meindert Sprang
> Every telephone cable in the country has the shield connected
> to ground at both ends.
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.
> 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.
And all those slave transmitters should be disabled, except the one that's
talking to the master.
> They are *all* high impedance devices, relatively. There has to
> be a 100 Ohm load resistor on each pair.
>
> >hard. However, both RS-232 and RS-422 can be used in a multidrop
> >configuration with a few diodes, but this would reduce the noise
> >margins, since the bus would only be actively driven into the Space
> >state, while passively pulled by bias resistors to the Mark (which
> >also is the idle state).
>
> Unnecessary for RS-422.
So, if all slaves have a dominant Mark, how is one slave going to drive a
Space on the line?
Meindert
Steve at fivetrees
> "Steve at fivetrees" <st...@NOSPAMTAfivetrees.com> wrote:
>>"Tim Mitchell" <ti...@sabretechnology.co.uk> wrote:
>>> RS485 is a 2-wire half duplex system where there is one transmitter and
>>> multiple receivers.
>>>
>>> RS422 is a 4-wire duplex system which goes between two devices, each of
>>> which has a transmitter and receiver.
>>
>>While this is essentially correct, I'd add:
>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>>actually 5-wire.
>
> RS-485 is a differential (balanced) system, and there is no
> signal ground connection. The cable used might well include a
> frame ground, but that is for noise induction cancellation, not
> signal ground.
>
> Hence it actually is a 2-wire or 4-wire link.
I'm a bit shocked at the amount of traffic this point has generated. It's
fairly fundamental.
If we were talking about balanced transformer-coupled audio, you'd be right.
But we're not - we're talking about common-mode voltage, which is
effectively +/-7V for RS-485 (on top of the 0-5V signal range, hence
+12/-7V). This (small) range is fine for connections that are physically
close, but really *not* fine for long lines across disparate grounds. (In
the old days of RS-422, I saw a lot of fried drivers for this very reason.)
In practice, one usually designs in an optically-isolated RS-485 interface,
to allow the 3rd/5th wire to be explicitly connected without
cross-connecting grounds.
So, I repeat: one *has* to consider common-mode. That 3rd or 5th wire must
be there, whether implicitly (via a common local ground) or explicitly (via
a physical cable).
Steve at fivetrees
news:vdu0b1hspo7iliauj...@4ax.com...
>> - RS-422 is also multidrop (10 max drops, IIRC).
>
> The RS-422 impedance levels allows for multiple receivers, but I do
> not see how a multidrop bidirectional system could be implemented
> within the RS-422 specification.
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.
Grant Edwards
>>> That is a frame ground, and not a signal ground. It will carry
>>> no signal current at all.
>>
>>It doesn't carry any signal current, but it is the ground to
>>which the receiver's input signal range specs are references.
>>It's the ground that defines what "0V" is for the signal
>>inputs. I call that the signal ground.
>
> It's a frame ground.
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.
> A signal ground would be a return path to ground for the
> signal. RS-232 has such a signal, and is "single ended",
> hence all signal lines share the same common ground return
> line.
>
> Differential ciruits are "balanced", the signal is between the
> two wires of a pair. Neither of them is at ground, so neither
> is called a "signal ground". The signal does not depend on
> any relationship to ground.
>
> If course, the receiver typically cannot tolerate a common mode
> voltage greater than some specified voltage. That that is not
> a signal voltage in any way. It just biases the devices out of
> their useful dynamic range.
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?
> Optical isolators are are nice because they have a
> significantly higher "useful dynamic range". Otherwise, the
> signal on the cable is still the same.
>>> And any variation of current seen will be strictly noise.
>>
>>What current?
>
> Induced current into that ground wire accompanying the signal
> pairs. That's why you don't want it to be one small wire if
> it in the same bundle as the signal pairs.
>
>>> The trick is to get the induction into the ground wire to
>>> then, in the cable between the ground wire and the signal
>>> pairs, cancel the induction into the signal cables.
Are you talking about "inducing" a DC voltage?
>>I really don't understand what you're talking about. The
>
> It is a bit complex. Many people who work with cables don't
> understand it very well. But any one who sits down and
> studies it a bit can understand it. It isn't even rocket
> science! ;-)
>
>>differential receiver inputs can deal with only a few volts of
>>common mode DC voltage. You have to use a ground that's common
>>between the transmitters and receivers to make sure that the
>>common-mode DC voltage seen by the receivers is within spec.
>
> Yes. You also have to be very careful about the currents
> induced into said ground connection. Do it the wrong way, and
> it adds noise to the signal pairs; do the right way and it
> will help cancel noise induced from the same source into those
> signal pairs.
What "noise"? I'm talking about controlling common-mode DC
level difference between the RS-485 transmitter and receiver.
>>> What kind of distances have you tried that with?
>>
>>A couple kilometers.
>
> That's a pretty good run for RS-485.
I think the spec is 10km for decent twisted pair and low baud
rates (<1M).
>>> I'd expect that across the room or around the bend might be
>>> just fine (and wouldn't be needed because the offset between
>>> the ground systems wouldn't be high enough to be a problem).
>>> But if this went down the road 3000-4000 feet, and you
>>> actually did get a ground offset high enough to be a problem,
>>> using a single wire in the same cable to equalize the ground
>>> potential should add enough noise to your cable run to make it
>>> a real problem.
>>
>>It didn't seem to.
>
> What kind of cable was this? Cable you installed, or telco
> cable?
Cable an electrician installed.
>>> A proper ground on each would be much better.
>>
>>Not allowed for safety reasons. The RS-485 transceivers at
>>both ends are optically isolated from earth.
>
> The cable still has to be grounded at both ends.
The shield may be grounded at one end or the other, but the
RS-485 common is not.
> That connects
> your two frame grounds together too.
>
>>> And a cable sheath that is properly grounded at *both* ends,
>>> to the same single point building ground that the RS-485
>>> equipment is tied to, would be the preferred way to make sure
>>> there wasn't too much common mode difference.
>>
>>Nope. The cable sheild is earth ground at one end or the other
>>and can't be electrically connected to the RS-485 signal or
>>"ground" signals.
>
> It *should* be connected to earth ground at both ends. And that
> ground point *should* be a single point where *all* frame
> grounds for the entire building go.
What are "frame grounds" and what do they have to do with the
RS-485 bus???
> Typically equipment bays in a single row are strapped together,
> though sometimes individual racks will have separate grounds.
> There should be a single cable from each row (or each rack if
> some racks are isolated) to a common grounding point on each
> floor of a building.
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.
> Each comm cable entrance would be considered just like an
> individually isolated rack, and would have its own ground
> cable going to the grounding point for that floor. (What this
> says is that the cable is *not* connected to a rack. That a
> rack in one row is *not* connected to a rack in a different
> row. That no two rows share a single cable going to the
> ground point. But often racks in one row share a ground
> cable, and often equipments mounted in one rack share ground
> wires.)
>
> Both ends of the cable should be grounded in that fashion.
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!!
visi.com
kunil
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
Grant Edwards
> 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.
Yes.
> But since RS485 is a differential mode protocol, we can use
> either A or B line as our common line.
I don't see how you can use A or B as a "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).
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.
> To make this "definite voltage level" same at the receiver /
> transceiver point, we need to earth their voltage reference
> node at the both side.
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!
visi.com
Floyd L. Davidson
Voice frequency circuits on twisted pair cable operate at *much* lower
voltage levels over *much* long distances, all without the consideration
you claim is necessary.
>So, I repeat: one *has* to consider common-mode. That 3rd or 5th wire must
>be there, whether implicitly (via a common local ground) or explicitly (via
>a physical cable).
You haven't caught the significance of what I've been saying either.
Your "3rd or 5th wire" is *not* the way to deal with ground potential
differences. What has been described causes more problems than it
cures.
Paul Keinanen
<st...@NOSPAMTAfivetrees.com> wrote:
>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.)
My point was that it is not mentioned in the standard.
>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.
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
Floyd L. Davidson
>"Floyd L. Davidson" <fl...@barrow.com> wrote:
>> >It would be unrealistic to assume that the grounding electrodes of two
>> >separate buildings would stay within the -7.. +12 V common mode range
>> >at all times (especially during thunderstorms), so optoisolation
>> >should be used to keep the grounds separate. The real question is, is
>> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
>> >a fiber optic cable be used instead.
>>
>> I don't have a lot of experience with thunderstorms, so I can't
>> really comment on that. Otherwise, that simply is not true.
>
>I have the same experience. I ran an RS-485 network in a factory invonment,
>using one twisted pair for data and two other pairs to distribute the power.
>Total length appr. 500 meters (1500 feet). Never ever had a problem, all
>boards shared the same power supply ground. When a link had to be made
>between two buildings, the power was split and only both datalines were
>connected. At both ends, the systems we ground. Every thunderstorm blew the
>Tx chips.
The obvious conclusion to be drawn is that it was not designed
properly, almost certainly due to a lack (then and now too
perhaps) of understanding about what caused the damage.
That has virtually *nothing* to do with signal ground, frame
grounds, power distribution, or for that matter common mode
voltages.
It has to do with induced voltages from static discharge
(lightening) and protection against such surges. You mentioned
nothing about that, and I assume there was none provided.
>> You *absolutely do* want a good ground connect at both ends.
>
>The only think you absolutely want is some means to prevent excessive common
>mode voltage outside the range of the TX chips. How you achieve this, is not
>important. This could easily be done with the "third wire" to connect both
>commons (I explicitly won't call it "ground").
It *is* important. And it isn't, as your example above
demonstrates, just common mode voltage equalization. This
"third wire" would have done *nothing* to benefit your situation
above.
The point is that if frame grounds are connected properly the
*normal* amount of noise on the cable is reduced. If done the
wrong way the noise will be increased. In fact, *either* way
will probably work most of the time! That leads to a lot of
statements to the effect of "we did that, and it worked". But
if you have a nationwide network, with millions of examples,
those which are done wrong do show up as being where 90% of the
maintenance costs go.
Floyd L. Davidson
>"Floyd L. Davidson" <fl...@barrow.com> wrote in message
>news:873brik...@barrow.com...
>> Every telephone cable in the country has the shield connected
>> to ground at both ends.
>
>I can assure you that the screen of my telephone cable is NOT connected to
>ground on my side.
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.
>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.
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.
>> 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.
>
>And all those slave transmitters should be disabled, except the one that's
>talking to the master.
They are high impedance devices. The only "disable" they need, is
to not be sending.
>> They are *all* high impedance devices, relatively. There has to
>> be a 100 Ohm load resistor on each pair.
>>
>> >hard. However, both RS-232 and RS-422 can be used in a multidrop
>> >configuration with a few diodes, but this would reduce the noise
>> >margins, since the bus would only be actively driven into the Space
>> >state, while passively pulled by bias resistors to the Mark (which
>> >also is the idle state).
>>
>> Unnecessary for RS-422.
>
>So, if all slaves have a dominant Mark, how is one slave going to drive a
>Space on the line?
If all of them try to send at one time, you get garble.
Paul E. Bennett
> "Meindert Sprang" <mhsp...@NOcustomSPAMware.nl> wrote:
>>"Floyd L. Davidson" <fl...@barrow.com> wrote:
>>> >It would be unrealistic to assume that the grounding electrodes of two
>>> >separate buildings would stay within the -7.. +12 V common mode range
>>> >at all times (especially during thunderstorms), so optoisolation
>>> >should be used to keep the grounds separate. The real question is, is
>>> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
>>> >a fiber optic cable be used instead.
Having designed a comms interface for one of the grottiest, noisiest
working environments I have ever come across and ensured that it would
survive the ordeal, I can confirm that management of the energy is where
you need to focus in order to prevent such failures occurring.
>>> I don't have a lot of experience with thunderstorms, so I can't
>>> really comment on that. Otherwise, that simply is not true.
You can get similar problems with a 20MA single turn secondary running from
a primary at 1kV which, when the circuit unexpectedly disrupts, releases
very large transients back through almost all the systems. Even the most
sensitive of our inputs (<10nV) is protected for at least 8.4kV short
duration transients. We only ever connect screens at one end (usually the
instrumentation cabinet) as, with that much energy around, the three
different earths can separate by as much as 75V for a short time. It is not
fair on cable screens to carry the sort of current that would imply (it
would probably cause a fire anyway).
>>I have the same experience. I ran an RS-485 network in a factory
>>invonment, using one twisted pair for data and two other pairs to
>>distribute the power. Total length appr. 500 meters (1500 feet). Never
>>ever had a problem, all boards shared the same power supply ground. When a
>>link had to be made between two buildings, the power was split and only
>>both datalines were connected. At both ends, the systems we ground. Every
>>thunderstorm blew the Tx chips.
You probably need to look at including more serial resistance and heavier
diode clamping at the intgerface connections. Also, control the impedance
of the connection of your power supply 0V to the chassis within each
module. Really think about the path that the surge/transient energy is
going to take.
> The obvious conclusion to be drawn is that it was not designed
> properly, almost certainly due to a lack (then and now too
> perhaps) of understanding about what caused the damage.
I'd concur with that conclusion.
> That has virtually *nothing* to do with signal ground, frame
> grounds, power distribution, or for that matter common mode
> voltages.
>
> It has to do with induced voltages from static discharge
> (lightening) and protection against such surges. You mentioned
> nothing about that, and I assume there was none provided.
Actually, you have to consider both the grounding issues and the
static/transient discharge issues in concert with each other. Some things
that may be very good for one are absolutely awful for the other. If you
need to minimise noise pick-up as well then you have quite a bit of work to
do. When I tell you to manage the energy, it is all of the energy you need
to consider (which includes your desired signal as well. PSpice can be a
tremendously good tool for sorting these issues out.
>>> You *absolutely do* want a good ground connect at both ends.
No, just the one end and preferrably the instrument rack/master end of the
cable. In the rare circumstance that you cannot isolate the screen at the
remote end put a break in the screen at a convenient point close to the
remote end and make sure it stays broken.
>>The only think you absolutely want is some means to prevent excessive
>>common mode voltage outside the range of the TX chips. How you achieve
>>this, is not important. This could easily be done with the "third wire" to
>>connect both commons (I explicitly won't call it "ground").
>
> It *is* important. And it isn't, as your example above
> demonstrates, just common mode voltage equalization. This
> "third wire" would have done *nothing* to benefit your situation
> above.
>
> The point is that if frame grounds are connected properly the
> *normal* amount of noise on the cable is reduced. If done the
> wrong way the noise will be increased. In fact, *either* way
> will probably work most of the time! That leads to a lot of
> statements to the effect of "we did that, and it worked". But
> if you have a nationwide network, with millions of examples,
> those which are done wrong do show up as being where 90% of the
> maintenance costs go.
By way of re-inforcing the point here, consider that the screen is only
meant to act as the notional extension of a metal enclosure out along the
wires. It should not carry any current at all (except maybe for the tiniest
leakage current capacitively coupled from the signal wires - and even that
should be miniscule). Dealing properly with the screens is a safety issue
as well as a circuit protection and noise reduction issue.
I shall have to look up the equipment build standard that details all these
issues and post the number. All embedded systems engineers should really
know this stuff anyway.
--
********************************************************************
Paul E. Bennett ....................<email://p...@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************
Paul Keinanen
Davidson) wrote:
>
>>> 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.
>>
>>And all those slave transmitters should be disabled, except the one that's
>>talking to the master.
>
>They are high impedance devices. The only "disable" they need, is
>to not be sending.
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
Floyd L. Davidson
>On 2005-06-16, Floyd L. Davidson <fl...@barrow.com> wrote:
>
>>>> That is a frame ground, and not a signal ground. It will carry
>>>> no signal current at all.
>>>
>>>It doesn't carry any signal current, but it is the ground to
>>>which the receiver's input signal range specs are references.
>>>It's the ground that defines what "0V" is for the signal
>>>inputs. I call that the signal ground.
>>
>> It's a frame ground.
>
>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.
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".
>> If course, the receiver typically cannot tolerate a common mode
>> voltage greater than some specified voltage. That that is not
>> a signal voltage in any way. It just biases the devices out of
>> their useful dynamic range.
>
>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?
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.
>>>> The trick is to get the induction into the ground wire to
>>>> then, in the cable between the ground wire and the signal
>>>> pairs, cancel the induction into the signal cables.
>
>Are you talking about "inducing" a DC voltage?
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.
>> Yes. You also have to be very careful about the currents
>> induced into said ground connection. Do it the wrong way, and
>> it adds noise to the signal pairs; do the right way and it
>> will help cancel noise induced from the same source into those
>> signal pairs.
>
>What "noise"? I'm talking about controlling common-mode DC
>level difference between the RS-485 transmitter and receiver.
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...
>>>> What kind of distances have you tried that with?
>>>
>>>A couple kilometers.
>>
>> That's a pretty good run for RS-485.
>
>I think the spec is 10km for decent twisted pair and low baud
>rates (<1M).
It was originally spec'd at 4000 feet. Better line driver
technology has extended that.
>> What kind of cable was this? Cable you installed, or telco
>> cable?
>
>Cable an electrician installed.
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.
>> The cable still has to be grounded at both ends.
>
>The shield may be grounded at one end or the other, but the
>RS-485 common is not.
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.
>> It *should* be connected to earth ground at both ends. And that
>> ground point *should* be a single point where *all* frame
>> grounds for the entire building go.
>
>What are "frame grounds" and what do they have to do with the
>RS-485 bus???
Ground that is not a signal path. E.g., common mode ground.
>> Typically equipment bays in a single row are strapped together,
>> though sometimes individual racks will have separate grounds.
>> There should be a single cable from each row (or each rack if
>> some racks are isolated) to a common grounding point on each
>> floor of a building.
>
>You keep talking about "frame grounds" and earth and stuff.
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.
>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.
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".
>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.
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.
Grant Edwards
>>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.
>
> What you are doing will result in equalizing the common mode DC
> offset from different grounds.
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.
> 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".
>
>>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.
>
> They should be.
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.
> 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).
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.
> On the other side, they are *all* connected to a common
> ground, if they are properly engineered.
--
Grant Edwards grante Yow! I'm having an
at emotional outburst!!
visi.com
Floyd L. Davidson
>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).
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).
>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.
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.
Meindert Sprang
> "Meindert Sprang" <mhsp...@NOcustomSPAMware.nl> wrote:
> >"Floyd L. Davidson" <fl...@barrow.com> wrote in message
> >news:873brik...@barrow.com...
> >> Every telephone cable in the country has the shield connected
> >> to ground at both ends.
> >
> >I can assure you that the screen of my telephone cable is NOT connected
to
> >ground on my side.
>
> 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.
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.
> Wrong. You want to have any current induced into each
> individual pair to be *more* significant in the shield than in
> the pair.
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.
> The shield, being grounded at both ends, will have
> current flow that will set up an *opposing* field to the
> original source,
Yes, but the pair inside the shield cannot "see" the original source, so
nothing is cancelled there.
> They are high impedance devices. The only "disable" they need, is
> to not be sending.
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.
> If all of them try to send at one time, you get garble.
Yes, that's why they have a control input to take the transmitter off line.
Meindert
Steve at fivetrees
news:d8sihj$ffk$1$830f...@news.demon.co.uk...
>
> By way of re-inforcing the point here, consider that the screen is only
> meant to act as the notional extension of a metal enclosure out along the
> wires. It should not carry any current at all (except maybe for the
> tiniest
> leakage current capacitively coupled from the signal wires - and even that
> should be miniscule). Dealing properly with the screens is a safety issue
> as well as a circuit protection and noise reduction issue.
Absolutely. Connecting to local ground at both ends of the run is a recipe
for HUGE ground currents. I've seen (very) melted ground wires. It is indeed
a safety hazard.
See my other post re isolated comms interfaces.
Paul E. Bennett
> 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
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.
Steve at fivetrees
news:87d5qmi...@barrow.com...
>>So, I repeat: one *has* to consider common-mode. That 3rd or 5th wire must
>>be there, whether implicitly (via a common local ground) or explicitly
>>(via
>>a physical cable).
>
> You haven't caught the significance of what I've been saying either.
>
> Your "3rd or 5th wire" is *not* the way to deal with ground potential
> differences. What has been described causes more problems than it
> cures.
I'm afraid I'm not just not catching you, I'm not following you at all.
Unless I'm missing something, you seem to be suggesting dangerous practices,
e.g. connecting a long cable to local ground at both ends. At that point I
started skipping your posts...
As I've just said elsewhere, this is simply nonsense. Consider: you're
trying to short out a part of the mains utility distribution system, which
may be carrying significant leakage currents over long distances, with a
flimsy little signal wire. Ground potential differences can be significant,
and the source impedance is very low indeed - certainly capable of
delivering tens of amps. I've seen guys who should know better staring at
melted cables, scratching their heads...
Please see my (and others') post(s) re isolated comms interfaces. Use one
ground; isolate from the other - i.e. keep both Tx and Rx relative to one
ground.
Steve at fivetrees
>>> 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.
>>
>>And all those slave transmitters should be disabled, except the one that's
>>talking to the master.
>
> They are high impedance devices. The only "disable" they need, is
> to not be sending.
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 at fivetrees
> On Thu, 16 Jun 2005 13:57:44 +0100, "Steve at fivetrees"
> <st...@NOSPAMTAfivetrees.com> wrote:
>
>>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.)
>
> My point was that it is not mentioned in the standard.
Point accepted.
>>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.
>
> A question of semantics, should such system be called a multidrop
> RS-422 system or a 4-wire RS-485 system :-).
Heh - this was from long before RS-485 was published ;).
> While this configuration is widely used, claiming that it conforms
> either to the RS-422 or RS-485 standard would be a bit suspicious.
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 at fivetrees
>
> You *absolutely do* want a good ground connect at both ends.
No, you absolutely *don't*. See my other posts.
Floyd L. Davidson
>Floyd L. Davidson wrote:
>>>> You *absolutely do* want a good ground connect at both ends.
>
>No, just the one end and preferrably the instrument rack/master end of the
>cable. In the rare circumstance that you cannot isolate the screen at the
>remote end put a break in the screen at a convenient point close to the
>remote end and make sure it stays broken.
That is *not* true in the situation being discussed, where there
is a cable long enough to have the two ends connected to separate
ground systems and equipment supplied by separate power systems.
See the other article I posted explaining it in detail. In
short, grounding at one end is an expedient design that works
for *short* cables within a single building.
>By way of re-inforcing the point here, consider that the screen is only
>meant to act as the notional extension of a metal enclosure out along the
>wires. It should not carry any current at all (except maybe for the tiniest
>leakage current capacitively coupled from the signal wires - and even that
>should be miniscule). Dealing properly with the screens is a safety issue
>as well as a circuit protection and noise reduction issue.
That is not correct. As noted above, see the other article, as
I'm not going to repeat it all in multiple posts.
>I shall have to look up the equipment build standard that details all these
>issues and post the number. All embedded systems engineers should really
>know this stuff anyway.
Telecommunications engineers, working with longer cable runs,
need to know even more... :-)
Steve at fivetrees
news:87zmtqg...@barrow.com...
>
> 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.
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 at fivetrees
> 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.
> be a 100 Ohm load resistor on each pair.
Errr... no, the 100R is there for transmission line matching. It should
match the characteristic impedance of the cable.
Floyd L. Davidson
>"Floyd L. Davidson" <fl...@barrow.com> wrote in message
>news:874qbyi...@barrow.com...
>> "Meindert Sprang" <mhsp...@NOcustomSPAMware.nl> wrote:
>> >"Floyd L. Davidson" <fl...@barrow.com> wrote in message
>> >news:873brik...@barrow.com...
>> >> Every telephone cable in the country has the shield connected
>> >> to ground at both ends.
>> >
>> >I can assure you that the screen of my telephone cable is NOT connected
>to
>> >ground on my side.
>>
>> 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.
>
>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.
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.
>> Wrong. You want to have any current induced into each
>> individual pair to be *more* significant in the shield than in
>> the pair.
>
>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.
See the other article I posted explaining that in detail.
>> The shield, being grounded at both ends, will have
>> current flow that will set up an *opposing* field to the
>> original source,
>
>Yes, but the pair inside the shield cannot "see" the original source, so
>nothing is cancelled there.
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.
>> They are high impedance devices. The only "disable" they need, is
>> to not be sending.
>
>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.
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.
>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.
That is only *one* way to accomplish that.
Steve at fivetrees
news:87ekb1h...@barrow.com...
> "Paul E. Bennett" <p...@amleth.demon.co.uk> wrote:
>>Floyd L. Davidson wrote:
>>>>> You *absolutely do* want a good ground connect at both ends.
>>
>>No, just the one end and preferrably the instrument rack/master end of the
>>cable. In the rare circumstance that you cannot isolate the screen at the
>>remote end put a break in the screen at a convenient point close to the
>>remote end and make sure it stays broken.
>
> That is *not* true in the situation being discussed, where there
> is a cable long enough to have the two ends connected to separate
> ground systems and equipment supplied by separate power systems.
You keep saying this, but it's wrong. If you won't believe us, use Google to
do some searching. A few seconds of searching turned up:
http://www.hw-server.com/rs232_signals.html
It's about RS-232, but the grounding issue under discussion is the same
(except it's worse with RS-232, since ground differences are seen as
signal). I'll quote a section:
>>
1 Protective Ground
Name: AA
Direction: -
CCITT: 101
This pin is usually connected to the frame of one of the devices, either the
DCE or the DTE, which is properly grounded. The sole purpose of this
connection is to protect against accidental electric shock and usually this
pin should not be tied to Signal Ground.
This pin should connect the chassis (shields) of the two devices, but this
connection is made only when connection of chassis grounds is safe (see
ground loops below) and it is considered optional.
Ground loops are low impedance closed electric loops composed from ground
conductors. When two grounded devices are connected together, say by a
RS-232 cable, the alternating current on the lines in the cable induces an
electric potential across the ends of the grounding line (either Protective
Ground or Signal Ground), and an electric current will flow across this line
and through the ground.
Since the loops impedance is low, this current can be quite high and easily
burn out electric components. Electrical storms could also cause a burst of
destructive current across such a loop. Therefore, connection of the
Protective Ground pin is potentially hazardous. Furthermore, not all signal
grounds are necessarily isolated from the chassis ground, and using a RS-232
interface, especially across a long distance, is unreliable and could be
hazardous. 30 meters is considered the maximum distance at which the
grounding signals can be connected safely.
<<
I repeat: what you keep proposing makes no sense. I can only presume that
you're confusing this issue with something else entirely.
Consider what happens when lightning strikes one location - local ground
potentials go *nuts*. Your (apparent) connection between two local ground
systems will try to connect them together, and it will lose. Bigtime.
You've mentioned telephone system cabling: I'm no expert, but I had thought
telephone cables were current-driven loops, with the power supplied by
a -48V supply and a local ground *at one end*. Perhaps this is only the
subscriber loop. For *very* long trunk cable runs, I have seen what you
describe - shield connected to local ground via a dirty great grounding
pole - but I had understood that the series impedance of the shield was
quite high, which is what saved it from becoming a very long and expensive
fuse. This is not what we're discussing here (the need for all RS-485
signals to be within a defined common-mode range).
Floyd L. Davidson
>On 2005-06-16, Floyd L. Davidson <fl...@barrow.com> wrote:
>
>>>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.
>>
>> What you are doing will result in equalizing the common mode DC
>> offset from different grounds.
>
>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.
I'm confused about what you actually have, see below.
>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.
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.
>> On the other side, they are *all* connected to a common
>> ground, if they are properly engineered.
--
CBFalconer
>
... snip ...
>
> 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.
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 (cbfal...@yahoo.com) (cbfal...@worldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!
Floyd L. Davidson
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
>news:87d5qmi...@barrow.com...
>>>So, I repeat: one *has* to consider common-mode. That 3rd or 5th wire must
>>>be there, whether implicitly (via a common local ground) or explicitly
>>>(via
>>>a physical cable).
>>
>> You haven't caught the significance of what I've been saying either.
>>
>> Your "3rd or 5th wire" is *not* the way to deal with ground potential
>> differences. What has been described causes more problems than it
>> cures.
>
>I'm afraid I'm not just not catching you, I'm not following you at all.
>Unless I'm missing something, you seem to be suggesting dangerous practices,
>e.g. connecting a long cable to local ground at both ends. At that point I
>started skipping your posts...
I've explained that *in detail*. That *is* the way telephone
cable systems are installed.
>As I've just said elsewhere, this is simply nonsense.
And you are out of your field...
>Consider: you're
>trying to short out a part of the mains utility distribution system, which
>may be carrying significant leakage currents over long distances, with a
>flimsy little signal wire. Ground potential differences can be significant,
>and the source impedance is very low indeed - certainly capable of
>delivering tens of amps. I've seen guys who should know better staring at
>melted cables, scratching their heads...
If they had a melted cable, it wasn't simply the difference in
ground potential. (Telecom cables generally are spliced and
grounded every 6000 feet, max.)
>Please see my (and others') post(s) re isolated comms interfaces.
See the tutorial I posted: Message-ID: <87vf4eg...@barrow.com>
>Use one
>ground; isolate from the other - i.e. keep both Tx and Rx relative to one
>ground.
That is fine within one building, and is not the best practice
for longer cable runs, for reasons explained in the message
listed above.
Floyd L. Davidson
>news:d8sihj$ffk$1$830f...@news.demon.co.uk...
>>
>> By way of re-inforcing the point here, consider that the screen is only
>> meant to act as the notional extension of a metal enclosure out along the
>> wires. It should not carry any current at all (except maybe for the
>> tiniest
>> leakage current capacitively coupled from the signal wires - and even that
>> should be miniscule). Dealing properly with the screens is a safety issue
>> as well as a circuit protection and noise reduction issue.
>
>Absolutely. Connecting to local ground at both ends of the run is a recipe
>for HUGE ground currents. I've seen (very) melted ground wires. It is indeed
>a safety hazard.
Bullshit.
>See my other post re isolated comms interfaces.
Why not do some real research, and find out how it is actually
done.
Grant Edwards
> EQUIP ISOLATOR RS485 <===/ CABLE /===> RS485 ISOLATOR EQUIP
>
> If you are doing that, with 10 km of cable
I don't think I ever saw runs longer than 2-3km.
> and *not* grounding the cable shield, it may well work,
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).
> 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.
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
visi.com rap session!!!
Floyd L. Davidson
Impedance is relative. To an 8 Ohm speaker, 600 Ohms is high impedance.
Floyd L. Davidson
>Floyd L. Davidson wrote:
>
>> 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
>
>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.
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.
Steve at fivetrees
>
>>As I've just said elsewhere, this is simply nonsense.
>
> And you are out of your field...
I've pointed out several factual errors with your posts. I've demonstrated
several areas where your understanding of RS-422/485 is, at best,
incomplete, and at worst, downright wrong. I (and many others) have provided
you with ample details of why what you're saying is just plain wrong.
"Out of your field", you say?
To be real clear about all this: I'm sure you're sincere, but you sound
confused. RS-422/485, and its practical applications, happens to be an area
I know *real* well. I'm a bit embarrassed for you.
Steve at fivetrees
>
> 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.
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.)
Floyd L. Davidson
>> That is *not* true in the situation being discussed, where there
>> is a cable long enough to have the two ends connected to separate
>> ground systems and equipment supplied by separate power systems.
>
>You keep saying this, but it's wrong. If you won't believe us, use Google to
I keep saying it because it is absolutely true.
>do some searching. A few seconds of searching turned up:
> http://www.hw-server.com/rs232_signals.html
>
>It's about RS-232, but the grounding issue under discussion is the same
>(except it's worse with RS-232, since ground differences are seen as
>signal). I'll quote a section:
In fact, that discussion is a subset, not at all similar to long
comm cables. It helps to have a good understanding about what
causes a "ground loop" and what causes noise induction in long
cables.
>1 Protective Ground
...
>Ground loops are low impedance closed electric loops composed from ground
>conductors. When two grounded devices are connected together, say by a
>RS-232 cable, the alternating current on the lines in the cable induces an
>electric potential across the ends of the grounding line (either Protective
>Ground or Signal Ground), and an electric current will flow across this line
>and through the ground.
Do you understand what that is saying? It's not exactly a good
example!
In fact, it's a bit of mumbo jumbo and clearly the person who
last edited it has no idea what a ground loop actually is! But
one little part almost got the point: "induces an electric
potential across the ends of the grounding line". That part is
right, but it is *not* from "alternating current on the lines in
the cable", and "induces" is the wrong term too.
Here's a ground loop:
Signal Source
o
|
Rload
|
+---------> connection =======//======= <------+
| to cable |
Rwire shield |
| |
| |
----- Earth Ground -----
--- ---
- -
Okay, there are three sources of current that affect voltages
across the two resistors. The "signal" is listed as "Signal
Source". Assume that is a current limited source, just to make
this more obvious. There is induced current in the cable
shield, and there is current from the ground potential
difference between the two Earth Grounds.
All currents contribute to the voltage across Rwire. The
voltage across Rload is affected by the voltage across Rwire,
and thus the voltage drop across Rwire and across Rload.
Current through the shield affecting the voltage across Rload is
noise. If that is significant, it is commonly said to be a
"ground loop". And obviously lifting the ground from the right
side will stop the current flow, and thus "cure" the problem.
However, so will reducing the resistance of Rwire, the common
ground wire. Hence you have two choices. One is very easy and
has no detrimental effects for *short* cables inside a building
that has both ends on the same AC power distribution and a
common ground or very low potential difference between two earth
grounds.
Long runs of comm cable do not fit that description, and
therefore use the technique allowing only *very low impedance*
common ground connections. (The only common part would have to
be very large, low impedance, cable that is preferably short.)
>I repeat: what you keep proposing makes no sense. I can only presume that
>you're confusing this issue with something else entirely.
It's simply good engineering, instead of magic.
(Where "magic" is defined as stuff that isn't understood
understand.)
>Consider what happens when lightning strikes one location - local ground
>potentials go *nuts*. Your (apparent) connection between two local ground
>systems will try to connect them together, and it will lose. Bigtime.
I mentioned not having a great deal of experience with
lightening. I should probably qualify that. I have only
a few years of experience with lightening, as opposed to
decades with the rest of this.
What you say above isn't true.
>You've mentioned telephone system cabling: I'm no expert, but I had thought
If you have no expertise in this topic, please *do not* pontificate.
>telephone cables were current-driven loops, with the power supplied by
>a -48V supply and a local ground *at one end*.
Wrong.
>Perhaps this is only the
>subscriber loop. For *very* long trunk cable runs, I have seen what you
>describe - shield connected to local ground via a dirty great grounding
>pole - but I had understood that the series impedance of the shield was
>quite high,
Wrong.
>which is what saved it from becoming a very long and expensive
>fuse. This is not what we're discussing here (the need for all RS-485
>signals to be within a defined common-mode range).
Wrong.
Steve at fivetrees
news:87zmtpe...@barrow.com...
>>You've mentioned telephone system cabling: I'm no expert, but I had
>>thought
>
> If you have no expertise in this topic, please *do not* pontificate.
You'll forgive me for laughing out loud here.
Floyd L. Davidson
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.
Steve at fivetrees
news:p5ydnfE1eqq...@pipex.net...
> "Floyd L. Davidson" <fl...@barrow.com> wrote in message
> news:87zmtpe...@barrow.com...
>>>You've mentioned telephone system cabling: I'm no expert, but I had
>>>thought
>>
>> If you have no expertise in this topic, please *do not* pontificate.
>
> You'll forgive me for laughing out loud here.
Figured I should clarify that.
Floyd, I'm really trying hard (now) to follow you. I've taken every diagram
you've drawn, viewed it with a fixed-width font, and tried hard to
understand your point. (Perhaps I'm misunderstanding something.) I've gone
back and re-read all your posts. I'm utterly convinced you're sincere,
you're passionate, and you know your stuff. I'm equally convinced that said
stuff does not include RS-422/485, or medium-haul datacomms in general. I
really believe you're missing the point (common-mode).
I'm also enjoying this thread. Keep 'em coming ;).
Steve at fivetrees
>>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.
>
> Like I said, low noise immunity. With a very low common mode
> dynamic range, for one thing.
Eh?
If you mean the +/-7V common-mode range, yeah, it's not huge - but it's more
than adequate if correctly applied.
Floyd L. Davidson
>news:87mzppg...@barrow.com...
>>
>> 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.
>
>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
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.
>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 ;).
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.
>(BTW, it's now 3:38am here in southern UK, and probably a bit warmer than
>where you are ;). Nice pics on your site.)
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
And I see you have no idea what the difference between a DS1 interface and
a RS-485 interface is when it comes to noise immunity.
As I said, check out a DS1 interface.
Floyd L. Davidson
>On 2005-06-17, Floyd L. Davidson <fl...@barrow.com> wrote:
>
>> EQUIP ISOLATOR RS485 <===/ CABLE /===> RS485 ISOLATOR EQUIP
>>
>> If you are doing that, with 10 km of cable
>
>I don't think I ever saw runs longer than 2-3km.
>
>> and *not* grounding the cable shield, it may well work,
>
>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---/ /--
> | +------+ /
> /// +--+--------/ /--
> |
> ///
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!
>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).
>
>> 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.
>
>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.
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
>>
>>>As I've just said elsewhere, this is simply nonsense.
>>
>> And you are out of your field...
>
>I've pointed out several factual errors with your posts. I've demonstrated
You have yet to point out a single factual error. Just *you*
saying it is, without supporting your comments with discussion
that explains a bit of it, is indicative.
I notice, for example, that you don't want to respond to the
detailed discussions I've provided on cable grounding. Instead
you pick other posts and just claim it isn't true. If you see
an error in the explainations I posted, please point it out.
I do make mistakes. I post when tired, I type too fast, and
don't always read it well enough. Plus sometimes I just have
the wrong idea. But you are going to have to support it with
more than pontification...
>several areas where your understanding of RS-422/485 is, at best,
>incomplete, and at worst, downright wrong. I (and many others) have provided
>you with ample details of why what you're saying is just plain wrong.
>
>"Out of your field", you say?
Apparently quit a ways out.
>To be real clear about all this: I'm sure you're sincere, but you sound
>confused. RS-422/485, and its practical applications, happens to be an area
>I know *real* well. I'm a bit embarrassed for you.
Could be Steve, but since the discussion has been about
*cables*, and *ground systems*, I don't see what value your
practical applications have to the topic when you don't seem
to understand the *specific* part that it is about.
Paul Keinanen
<st...@NOSPAMTAfivetrees.com> wrote:
>Absolutely. Connecting to local ground at both ends of the run is a recipe
>for HUGE ground currents. I've seen (very) melted ground wires. It is indeed
>a safety hazard.
Yes, this is a severe problem when the TN-C wiring system (L1, L2, L3
and N) in which the neutral is connected directly to each equipment
frame. In this case quite a few Amperes can flow in the cable shield.
However, with TN-S wiring (L1, L2, L3, N and PE) with the PE connected
to each frame, grounding the data cable at both end is not so bad,
since large AC currents will flow in the cable shield only during
ground faults (Lx to PE) until the fuse is blown.
Unfortunately pure TN-S systems are hard to find, since often the N,
PE, the grounding electrode and all metal piping is connected to the
same main grounding bar (equipotential bonding) close to the mains
cable entry. From the main grounding bar to the distribution
transformer, the system is just TN-C.
Thus, most wiring systems claiming to be TN-S are actually TN-C-S,
thus grounding the data cable shield at both ends would not be that
bad _within_ the building, but connecting a data cable shield between
two buildings, will suffer from the same huge currents as a TN-C
system would.
Paul
Paul Keinanen
Davidson) wrote:
>
>Here's a ground loop:
>
> Signal Source
>
> o
> |
> Rload
> |
> +---------> connection =======//======= <------+
> | to cable |
> Rwire shield |
> | |
> | |
> ----- Earth Ground -----
> --- ---
> - -
In the TN-C electric distribution system, the "Signal source" is the
mains phase voltage (100-240 V depending on country), Rload perhaps
10-1000 ohms, Rwire perhaps 0.2 ohm. An AC current up to 25 A could
flow though Rload and through Rwire. The voltage drop across Rwire
would be up to 5 V.
Before connecting the data cable, there would be a 5 V ground
potential difference between the left and right side equipment. When
the cable shields are connected, a part of the up to 25 A flowing
through Rload will be diverted through the data cable and into the
right hand equipment ground.
The magnitude of this "ground loop" current depends on the ratio of
the Rwire compared to the combined resistance of the cable shield and
the right hand side equipment grounding resistance. Especially, if the
right hand side equipment is close to the main distribution panel, the
total path resistance is dictated by the shield resistance. If the
alternate path total resistance is as low as Rwire, more than 10 A
could flow in the cable shield, which could even cause a fire hazard.
Paul
CBFalconer
> fl...@barrow.com (Floyd L. Davidson) wrote:
>
>> Here's a ground loop:
>>
>> Signal Source
>>
>> o
>> |
>> Rload
>> |
>> +---------> connection =======//======= <------+
>> | to cable |
>> Rwire shield |
>> | |
>> | |
>> ----- Earth Ground -----
>> --- ---
>> - -
>
> In the TN-C electric distribution system, the "Signal source" is
> the mains phase voltage (100-240 V depending on country), Rload
> perhaps 10-1000 ohms, Rwire perhaps 0.2 ohm. An AC current up to
> 25 A could flow though Rload and through Rwire. The voltage drop
> across Rwire would be up to 5 V.
You should also realize that 'signal source' above includes a
myriad of possibilities, including lightning, pools of acid forming
batteries, whatever. We can never really get at a true ground
except through some sort of impedance. Luckily most (but not all)
of our controllable signal sources are imposed between the marked
signal point and the junction of Rwire and Rload.
Also the effects of Rwire, and the IR drop across it, can be
present at either the near or far end, or both. Those R things are
not necessarily resistive, nor passive.
Grant Edwards
>>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---/ /--
>> | +------+ /
>> /// +--+--------/ /--
>> |
>> ///
>
>
> 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!
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).
>> 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.
>
> 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.
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
visi.com RUBIK's CUBE HANDBOOK FOR
DEAD CAT LOVERS"...
Steve at fivetrees
news:873brhe...@barrow.com...
> "Steve at fivetrees" <st...@NOSPAMTAfivetrees.com> wrote:
>>"Floyd L. Davidson" <fl...@barrow.com> wrote:
>>>
>>>>As I've just said elsewhere, this is simply nonsense.
>>>
>>> And you are out of your field...
>>
>>I've pointed out several factual errors with your posts. I've demonstrated
>
> You have yet to point out a single factual error. Just *you*
> saying it is, without supporting your comments with discussion
> that explains a bit of it, is indicative.
<snip>
> But you are going to have to support it with
> more than pontification...
I just looked up the dictionary definition of "pontification". It has two
meanings, but I'm ignoring one, as I have no immediate plans to become the
next pope. The other involves opinions.
The only opinion I've expressed in this thread is that you're misguided.
Everything I've said about RS-422/485 is factual, and you are very welcome
to check these facts against the published specs. (I have them here, and am
very familiar with them. You don't appear to have read them.)(I can't point
you at them on the web; the specs are not in the public domain.)
Equally, everything I've said about double-grounding is also factual, and
demonstrable. I'm not aware of any "specs" as such on this issue, but it is
hard to argue with a melted signal cable.
Finally, look around. Do you see anyone agreeing with your somewhat strange
point of view? Does this not tell you something?
Whatever it is you're talking about, it's certainly not RS-422/485.
Floyd L. Davidson
>On Thu, 16 Jun 2005 18:49:55 -0800, fl...@barrow.com (Floyd L.
>Davidson) wrote:
>
>>
>>Here's a ground loop:
>>
>> Signal Source
>>
>> o
>> |
>> Rload
>> |
>> +---------> connection =======//======= <------+
>> | to cable |
>> Rwire shield |
>> | |
>> | |
>> ----- Earth Ground -----
>> --- ---
>> - -
>
>In the TN-C electric distribution system, the "Signal source" is the
>mains phase voltage (100-240 V depending on country), Rload perhaps
>10-1000 ohms, Rwire perhaps 0.2 ohm. An AC current up to 25 A could
>flow though Rload and through Rwire. The voltage drop across Rwire
>would be up to 5 V.
No.
"Signal Source" means whatever your *desired* signal is. Not
the AC power distribution... simply because how much that is
perterbed is of no significance.
Across Rload... is your Hifi Amp, your telephone set, the PA
system, computer data, gas tank level sensor output, whatever...
However, we could add that and many other sources of voltages.
It makes the diagram and the discussion more complex, but the
one particular example you chose is an excellent one because
it is commonly seen and often has significant effect.
>Before connecting the data cable, there would be a 5 V ground
>potential difference between the left and right side equipment. When
>the cable shields are connected, a part of the up to 25 A flowing
>through Rload will be diverted through the data cable and into the
>right hand equipment ground.
But lets not suggest that it would be a significant part of that
25 A. We are *not* describing ground *faults*, but ground
*loops*! But, in a typical arrangement, it could certainly have
enough effect to cause one heck of a 60 Hz hum in the "signal"
as described above.
>The magnitude of this "ground loop" current depends on the ratio of
>the Rwire compared to the combined resistance of the cable shield and
>the right hand side equipment grounding resistance. Especially, if the
>right hand side equipment is close to the main distribution panel, the
>total path resistance is dictated by the shield resistance. If the
>alternate path total resistance is as low as Rwire, more than 10 A
>could flow in the cable shield, which could even cause a fire hazard.
We need to make the distinction that this is a ground system fault,
not an inherent characteristic of the indicated connections. And
it can happen whether the arrangement is properly designed or not.
Floyd L. Davidson
>On Fri, 17 Jun 2005 00:22:38 +0100, "Steve at fivetrees"
><st...@NOSPAMTAfivetrees.com> wrote:
>
>>Absolutely. Connecting to local ground at both ends of the run is a recipe
>>for HUGE ground currents. I've seen (very) melted ground wires. It is indeed
>>a safety hazard.
I hate to mention this, but there is a difference between comm cable
and power cable. The Neutral in three phase power systems is *not*
the equivalent of the shield on a comm cable.
>Yes, this is a severe problem when the TN-C wiring system (L1, L2, L3
>and N) in which the neutral is connected directly to each equipment
>frame. In this case quite a few Amperes can flow in the cable shield.
>
>However, with TN-S wiring (L1, L2, L3, N and PE) with the PE connected
>to each frame, grounding the data cable at both end is not so bad,
>since large AC currents will flow in the cable shield only during
>ground faults (Lx to PE) until the fuse is blown.
>
>Unfortunately pure TN-S systems are hard to find, since often the N,
>PE, the grounding electrode and all metal piping is connected to the
>same main grounding bar (equipotential bonding) close to the mains
>cable entry. From the main grounding bar to the distribution
>transformer, the system is just TN-C.
>
>Thus, most wiring systems claiming to be TN-S are actually TN-C-S,
>thus grounding the data cable shield at both ends would not be that
>bad _within_ the building, but connecting a data cable shield between
>two buildings, will suffer from the same huge currents as a TN-C
>system would.
>
>Paul
--
Floyd L. Davidson
>Equally, everything I've said about double-grounding is also factual, and
>demonstrable. I'm not aware of any "specs" as such on this issue, but it is
>hard to argue with a melted signal cable.
So you declare yourself correct, eh? No discussion, no facts, nothing
but pontification.
If it is demonstrable, why don't you!
Steve at fivetrees
> "Steve at fivetrees" <st...@NOSPAMTAfivetrees.com> wrote:
>>Equally, everything I've said about double-grounding is also factual, and
>>demonstrable. I'm not aware of any "specs" as such on this issue, but it
>>is
>>hard to argue with a melted signal cable.
>
> So you declare yourself correct, eh? No discussion, no facts, nothing
> but pontification.
I repeat: please see the RS-422/485 specs.
> If it is demonstrable, why don't you!
I, and others, have been trying. I'm about ready to give up.
Lanarcam
IMHO, This is related to the difference between mechanical
and electronic ground. In the systems we made years ago,
the electronic ground was isolated from the mechanical
ground. A jumper was provided that enabled the customer
to tie both grounds if he wished. He was responsible
for the earthing of the system.
You will find perhaps the following useful:
(http://www.spheresystems.com.au/RS485.html)
Ground and Earth Connections
The grounding and earthing connections in RS485 provide two
separate functions.
The second is related to safety and the first to establish a
reference voltage
Safety
The RS485 *cable screen* must be bonded to the protective earth
system of a building at *one point* only.
The cable screen must be electrically continuous throughout the
entire cable run
Voltage Reference
The screen of the RS485 cable establishes a ground reference
voltage for the RS485 signal conductors. For this reason the
cable shield must be connected to the *ground reference* for
*each node* on the network.
It is not acceptable practice to tie the node ground reference
to the building protective earth as this will introduce
electrical noise into the system and may lead to equipment
damage in the event of electrical fault currents
Floyd L. Davidson
>On Fri, 17 Jun 2005 09:32:32 -0800, fl...@barrow.com (Floyd L.
>Davidson) wrote:
>
>>>Before connecting the data cable, there would be a 5 V ground
>>>potential difference between the left and right side equipment. When
>>>the cable shields are connected, a part of the up to 25 A flowing
>>>through Rload will be diverted through the data cable and into the
>>>right hand equipment ground.
>>
>>But lets not suggest that it would be a significant part of that
>>25 A. We are *not* describing ground *faults*, but ground
>>*loops*! But, in a typical arrangement, it could certainly have
>>enough effect to cause one heck of a 60 Hz hum in the "signal"
>>as described above.
>
>used in the world. The TN-S system with separate N and PE conductors
>is not the only system used.
I'm certainly *not* a power engineer.
>While it is true that in a TN-S system, large currents will flow in
>the signal cable shields only in ground fault situations, in the TN-C
>system with a common PEN conductor, large currents can flow in the
>data cable shield in normal conditions. Based on the numerous posts,
>it appears that you are completely ignorant of this wiring practice
>used in many parts of the world.
>
>Even the US wiring system appears to be some kind of TN-C-S with
>separate neutral and ground within the house, but a common PEN
>conductor to the distribution transformer. This system also suffers
>from large data cable shield currents when connecting two buildings,
>if both ends are grounded.
You don't seem to be a comm engineer.
As I've noted many times, telco comm cables are grounded at both
ends.
Paul Keinanen
Davidson) wrote:
>>Before connecting the data cable, there would be a 5 V ground
>>potential difference between the left and right side equipment. When
>>the cable shields are connected, a part of the up to 25 A flowing
>>through Rload will be diverted through the data cable and into the
>>right hand equipment ground.
>
>But lets not suggest that it would be a significant part of that
>25 A. We are *not* describing ground *faults*, but ground
>*loops*! But, in a typical arrangement, it could certainly have
>enough effect to cause one heck of a 60 Hz hum in the "signal"
>as described above.
You seem to have a very limited view of various grounding practices
used in the world. The TN-S system with separate N and PE conductors
is not the only system used.
While it is true that in a TN-S system, large currents will flow in
the signal cable shields only in ground fault situations, in the TN-C
system with a common PEN conductor, large currents can flow in the
data cable shield in normal conditions. Based on the numerous posts,
it appears that you are completely ignorant of this wiring practice
used in many parts of the world.
Even the US wiring system appears to be some kind of TN-C-S with
separate neutral and ground within the house, but a common PEN
conductor to the distribution transformer. This system also suffers
from large data cable shield currents when connecting two buildings,
if both ends are grounded.
Paul
Floyd L. Davidson
>Steve at fivetrees wrote:
>You will find perhaps the following useful:
>(http://www.spheresystems.com.au/RS485.html)
>
>Ground and Earth Connections
>The grounding and earthing connections in RS485 provide two
>separate functions.
>
>The second is related to safety and the first to establish a
>reference voltage
>
>Safety
>The RS485 *cable screen* must be bonded to the protective earth
>system of a building at *one point* only.
>
>The cable screen must be electrically continuous throughout the
>entire cable run
>
>Voltage Reference
>
>The screen of the RS485 cable establishes a ground reference
>voltage for the RS485 signal conductors. For this reason the
>cable shield must be connected to the *ground reference* for
>*each node* on the network.
>
>It is not acceptable practice to tie the node ground reference
>to the building protective earth as this will introduce
>electrical noise into the system and may lead to equipment
>damage in the event of electrical fault currents
For Steve's benefit, you should have included one more section:
Installation throughout Multiple Buildings
There are two separate installation procedures depending on
the type of electrical earthing system.
MEN System at one building
In this case the installation may be made as though it
exists in a single building. The cable shield will be
continuous throughout the installation.
MEN System in each building
In this case there must be electrical isolation between
buildings with different MEN systems. Each building is
wired as a separate and complete installation with the
cable shield tied to the building protective earth at one
point.
That is in *each* building.
Floyd L. Davidson
>> "Steve at fivetrees" <st...@NOSPAMTAfivetrees.com> wrote:
>>>Equally, everything I've said about double-grounding is also factual, and
>>>demonstrable. I'm not aware of any "specs" as such on this issue, but it
>>>is
>>>hard to argue with a melted signal cable.
>>
>> So you declare yourself correct, eh? No discussion, no facts, nothing
>> but pontification.
>
>I repeat: please see the RS-422/485 specs.
Please quote where those specs discuss grounding of comm cables,
or explain what ground loops are, or in support any of the other
invalid pronouncements you've been prone to making.
Paul E. Bennett
> "Steve at fivetrees" <st...@NOSPAMTAfivetrees.com> wrote:
>>"Paul E. Bennett" <p...@amleth.demon.co.uk> wrote in message
>>news:d8sihj$ffk$1$830f...@news.demon.co.uk...
>>>
>>> By way of re-inforcing the point here, consider that the screen is only
>>> meant to act as the notional extension of a metal enclosure out along
>>> the wires. It should not carry any current at all (except maybe for the
>>> tiniest
>>> leakage current capacitively coupled from the signal wires - and even
>>> that should be miniscule). Dealing properly with the screens is a safety
>>> issue as well as a circuit protection and noise reduction issue.
>>
>>Absolutely. Connecting to local ground at both ends of the run is a recipe
>>for HUGE ground currents. I've seen (very) melted ground wires. It is
>>indeed a safety hazard.
>
> Bullshit.
When you deal with the MEGA-AMP power world you will get to see how much of
a safety hazard connecting the screens at both ends is. Consider that,
under certain circumstances there may exist 75 to 80 Volts between the two
earths and that the current source is huge by comparison to the screen
current carrying capacity. If we assume that the run of cable has a screen
resistance of about 1.5 Ohms (a reasonable figure for quite a 150m run of
decent signal cable) then 80V * 80V / 1.5 Ohms gives you a power
dissipation of 4.267kW along the length of the screen. So, you had better
limit the current flow in the screen in some manner or you fry the signal
cable.
>>See my other post re isolated comms interfaces.
>
> Why not do some real research, and find out how it is actually
> done.
Many of us have and employ this sort of consideration in our daily practice.
--
********************************************************************
Paul E. Bennett ....................<email://p...@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************
Paul E. Bennett
Each installation does need to be examined and understood. I know that our
earthing scheme is split three ways. There is building earth which is the
main safety earth for the entire building. We also have an instrumentation
earth which is a quite large (10mm) cable from an instrument earth busbar
around the building and linked to building earth in certain specific
locations. Finally we have machine earth which is associated with the
MEGA-Amp equipment that we use in our experiments and which only connects
to building earth at certain specific points. During experimental
operations we can see differences between the machine earth and the
instrument earth that are of the order of 75 to 80V. If you need to work on
some of the equipment during operational periods we have to write a safe
working procedure fill out a permit to work form and get it all signed by a
large number of people before we are allowed near the equipment. Sometimes
it can take a couple of days before you are allowed to repair some
measurement equipment.
Paul E. Bennett
> I've explained that *in detail*. That *is* the way telephone
> cable systems are installed.
If you are speaking of purely telephone wires then most cables I have seen
in that category are not screened. Just twisted pairs. However, as I do not
work in telephones I will bow to yours and anyone elses knowledge of that
aspect (I could probably ask my brother who works with telephones as well
as pwer systems in his daily work).
>>As I've just said elsewhere, this is simply nonsense.
>
> And you are out of your field...
I am certainly not in telephony.
>>Consider: you're
>>trying to short out a part of the mains utility distribution system, which
>>may be carrying significant leakage currents over long distances, with a
>>flimsy little signal wire. Ground potential differences can be
>>significant, and the source impedance is very low indeed - certainly
>>capable of delivering tens of amps. I've seen guys who should know better
>>staring at melted cables, scratching their heads...
>
> If they had a melted cable, it wasn't simply the difference in
> ground potential. (Telecom cables generally are spliced and
> grounded every 6000 feet, max.)
I know that on navy ships and submarines they connect the outer screen to
the feedthrough grounds as they penetrate each bulkhead. There are special
feedthrough glands made for this purpose. Then, they also go through a lot
of effort to ensure that the electrical ground really is as close to ship
ground as possible (usually less than 250mV). A ship or submarine can be
considered akin to one building with own power source, however, the cable
runs are quite long.
>>Please see my (and others') post(s) re isolated comms interfaces.
>
> See the tutorial I posted: Message-ID: <87vf4eg...@barrow.com>
>
>>Use one
>>ground; isolate from the other - i.e. keep both Tx and Rx relative to one
>>ground.
>
> That is fine within one building, and is not the best practice
> for longer cable runs, for reasons explained in the message
> listed above.
I would hope that it is obvious to all from this discussion that the issue
of connecting screens/shields is rather dependent on the circumstances that
the equipment is going into. The screen connection impedance idea that I
mentioned in my earlier post would probably work for the widest range of
different situations but I would still countenance understanding the issues
that surround each individual instalation. It can save a huge amount of
grief later.
Floyd L. Davidson
>Floyd L. Davidson wrote:
>>>Absolutely. Connecting to local ground at both ends of the run is a recipe
>>>for HUGE ground currents. I've seen (very) melted ground wires. It is
>>>indeed a safety hazard.
>>
>> Bullshit.
>
>When you deal with the MEGA-AMP power world you will get to see how much of
And since we are *not* dealing with a power distribution system,
your point seems to be less than appropriate.
>a safety hazard connecting the screens at both ends is. Consider that,
>under certain circumstances there may exist 75 to 80 Volts between the two
>earths and that the current source is huge by comparison to the screen
>current carrying capacity. If we assume that the run of cable has a screen
>resistance of about 1.5 Ohms (a reasonable figure for quite a 150m run of
>decent signal cable) then 80V * 80V / 1.5 Ohms gives you a power
>dissipation of 4.267kW along the length of the screen. So, you had better
>limit the current flow in the screen in some manner or you fry the signal
>cable.
>
>>>See my other post re isolated comms interfaces.
>>
>> Why not do some real research, and find out how it is actually
>> done.
>
>Many of us have and employ this sort of consideration in our daily practice.
Reality check time: telephone cables all across the country and
around the world are grounded every 3000 or 6000 feet.
Jim Stewart
> Reality check time: telephone cables all across the country and
> around the world are grounded every 3000 or 6000 feet.
>
Not in the usage of the term "grounded" that
I'm familiar with. Perhaps you could give us
a little more details.
Floyd L. Davidson
>
>> I've explained that *in detail*. That *is* the way telephone
>> cable systems are installed.
>
>If you are speaking of purely telephone wires then most cables I have seen
>in that category are not screened. Just twisted pairs. However, as I do not
Telephone cable, the stuff on the poles or buried in a trench,
that goes between buildings and on down the road, has a shield.
(Heck, some cables have six inches of armor plated steel around
them!)
Cross connect cables between bays does, and between racks does
not.
Drop wire doesn't. Frame wire doesn't.
>I am certainly not in telephony.
Then you should not propose pseudo-authoritative answers to
questions relating to that field.
>> If they had a melted cable, it wasn't simply the difference in
>> ground potential. (Telecom cables generally are spliced and
>> grounded every 6000 feet, max.)
>
>I know that on navy ships and submarines they connect the outer screen to
>the feedthrough grounds as they penetrate each bulkhead. There are special
>feedthrough glands made for this purpose. Then, they also go through a lot
>of effort to ensure that the electrical ground really is as close to ship
>ground as possible (usually less than 250mV). A ship or submarine can be
>considered akin to one building with own power source, however, the cable
>runs are quite long.
The longest run on an aircraft carrier, or a super tanker, is
short. More than 3 miles is considered "long" in general,
though specifically that may not always be the case.
>> That is fine within one building, and is not the best practice
>> for longer cable runs, for reasons explained in the message
>> listed above.
>
>I would hope that it is obvious to all from this discussion that the issue
>of connecting screens/shields is rather dependent on the circumstances that
You've now mentioned ships and "MEGA-Amp equipment". There are
many special cases, probably none of which lead to any
enlightenment except when they are explicitly discussed as
extremes and flagged as unusual.
I could also mention a few odd places... the Faraday enclosure
for radio and carrier equipment at a Coast Guard Loran station.
Anything near a large power generating facility. The "tempest"
shielding into a room where the USAF was doing super secret
seismic analysis (to detect nuclear testing in Siberia). Or the
operation of electronic in proximity to DEWLINE or Long Range
Radar sites. For that matter a Toll Center is one of the
harshest electrical noise environments around.
These are all wonderful stories and any time you want to swap
tales, we can. Using them to try to explain grounding on comm
cables won't help anyone understand what is involved though.
>the equipment is going into. The screen connection impedance idea that I
>mentioned in my earlier post would probably work for the widest range of
>different situations but I would still countenance understanding the issues
>that surround each individual instalation. It can save a huge amount of
>grief later.
Understanding the basics comes first. Trying to extrapolate
these extreme situations to be representative of the basic
theory is exactly what causes so much confusion.
Floyd L. Davidson
At every point where sections of cable is spliced, the
shield on both sides of the splice is bonded, and grounded
to an earth ground. That is generally a copper rod driven
into the ground.
Paul E. Bennett
See
<http://www.svconline.com/news/avinstall_implementing_electronicsystems_wiringsignal/>
There is currently some discussion on AES48.
<http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>
This document is on Industrial Ethernet.
http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/Belden.pdf
This one is a facilities departments standard.
<http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120
Cable Wire and Connectors 600V.doc>
So, there is plenty of material there that states that the signal cable
shields shall only be connected at one end. Like I have stated before, each
situation should be carefully considered, calculations performed, safety
assessments performed (yes, even for the simple audio lead between intercom
units) and the complet circumstances fuly understood before you go in with
a half baked notion of what is right. Circumstances can change things quite
drastically. You just need to understand the underlying physics of the
situation a bit.
Paul E. Bennett
> Here's a ground loop:
>
> Signal Source
>
> o
> |
> Rload
> |
> +---------> connection =======//======= <------+
> | to cable |
> Rwire shield |
> | |
> | |
> ----- Earth Ground -----
> --- ---
> - -
>
>
> Okay, there are three sources of current that affect voltages
> across the two resistors. The "signal" is listed as "Signal
> Source". Assume that is a current limited source, just to make
> this more obvious. There is induced current in the cable
> shield, and there is current from the ground potential
> difference between the two Earth Grounds.
OK, we will start by assuming that the Earth Ground potential at the left
and right ends are exactly the same, just for starters. We will also assume
that the Rwire for the right hand end is the same as the Rwire at the left
hand end.
Current flow in the cable shield will be roughly (0.5*Rwire)/Rload. If
Rload was a few kOhms and Rwire was nearer 0.01 Ohms then there is almost a
negligable current flow.
Now consider the case where the left hand ground moves up to 80V away from
the right hand ground, but that the impedance between the two ground
connections remains less than 10 Ohms (not unreasonable in some very large
buildings). I am sure that you can see why we have been advocating the
grounding at one end only rule as a safety aspectg of RS422/RS485 networks,
as well as almost any other low level signal measurement/management system.
Where you do need to connect the screen wire to both ends then you had
better include some impedance in the connection that limits dangerous
current flow and yet provides sufficient coupling to ground for the higher
frequency signals. As I have stated before, all the circumstances have to
be well understood to make the right choice.
Paul E. Bennett
> This message is a courtesy copy of a Usenet article posted to:
> comp.arch.embedded
> --------------------------------------------------------------
>
> "Paul E. Bennett" <p...@amleth.demon.co.uk> wrote:
>>Floyd L. Davidson wrote:
>>
>>> 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
>>
>>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.
>
> 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.
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).
> Keep in mind that the whole idea is to allow the current flow to
> generate an equal and opposite induction into the signal pairs.
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.
Paul E. Bennett
> And since we are *not* dealing with a power distribution system,
> your point seems to be less than appropriate.
It is more pertinent than you realise. The equipment I deal with
incorporates a range of measurement and control systems that includes
nano-volt signals, RS485 signals and 4-20mA current loop signals passed
over 150m of multi-core shielded twisted pair cable to the instrument
racks. The environment that this lives with is extremely noisy, features
high magnetic fields, extremes of temperature range and earth potentials
that can leap well off the normal reference level (and it is not the
noisiest environment I have worked with). Some of the communications
signals run in quieter areas and span about 50 different buildings on the
site. Each building has its own sub-station and the site has a SuperGrid
power feed (it uses short bursts of extremely high power at regular
intervals). So, as you can imagine, as a Systems Engineer with
responsibility for just some of the equipment on this site, I have to know
how the site earthing scheme works and what risks are involved. So in all
respects I am quite on topic for this thread and relevent with the advice I
have provided.
>>>>See my other post re isolated comms interfaces.
>>>
>>> Why not do some real research, and find out how it is actually
>>> done.
>>
>>Many of us have and employ this sort of consideration in our daily
>>practice.
>
> Reality check time: telephone cables all across the country and
> around the world are grounded every 3000 or 6000 feet.
Last time I checked around here the telephone company cables were
unshielded twisted pairs (but that was quite some while ago).
The other types of cable we use on site include unshielded and shielded
power cables, heliax coaxial cables and triaxial cables. I have to know how
to deal with each and every type.
Floyd L. Davidson
>
><http://www.svconline.com/news/avinstall_implementing_electronicsystems_wiringsignal/>
This one starts out pretty good, when discussing theory, and fails
terribly when discussing implementations. If nothing else, the
discussion of jack fields should be enough to indicate the confusion.
Regardless, nothing there contradicts what I've said. The entire
discussion is about cables within a single structure, ground system
and power distribution. It also appears to be discussing single
pair cables for the most part.
>There is currently some discussion on AES48.
>
><http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>
IEC 61000-5-2, 6.4.2, points out, "Shields of
cables are bonded to the earthing network at one or
two extremities depending on the signals being
transmitted and on possible electromagnetic
interference sources."
However, despite contradicting *you*, the context is not
complete enough to actually know *what* they are referring to.
And casual discussion, much like those on this newsgroup, are
not authoritative sources.
>This document is on Industrial Ethernet.
>
>http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/Belden.pdf
This one contradicts your first cite above in regard to the
effects of a ground loop. Shame on Belden. It is also talking
only about short distance ethernet. It would make no sense at
all to ground both ends of an ethernet cable. Or, more correctly,
it would make no sense to attempt using ethernet over a cable where
grounding at both ends was useful.
>This one is a facilities departments standard.
>
><http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120
>Cable Wire and Connectors 600V.doc>
This is also talking only about relatively short runs of
"control cable" all within a building, all on one ground system,
and all where both ends are on the same power distribution.
Grounding only one end of a cable, in those circumstances is
expedient.
>So, there is plenty of material there that states that the signal cable
>shields shall only be connected at one end.
No, that is *NOT* what your sources above indicate. They do
indicate that is some circumstances (which I've already pointed
out in previous articles), the easiest means of avoiding trouble
with ground loops is simply to ground at only one end. That is
not a "shall only" situation at all, nor is that necessarily
the best solution.
>... You just need to understand the underlying physics of the
>situation a bit.
And we have yet to see *any* indication that you do.
Here, try reading these...
Here's a blurb on equipment to test the ground and shield
continuity at comm cable splices (where you are saying grounds
don't exist).
http://www.tequipment.net/WilcomTeleTransT124.asp
Here are a couple of specifications for terminating cables
to building ground systems:
http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuidelines/16743%20-%20Telecom%20Risers,%20Closets%20and%20Entries.htm
Here's short URL for that last one,
Here's an even more authoritative treatment of grounding,
http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc
All of these describe the grounding at *both* ends of a cable.
Paul E. Bennett
> Telephone cable, the stuff on the poles or buried in a trench,
> that goes between buildings and on down the road, has a shield.
> (Heck, some cables have six inches of armor plated steel around
> them!)
>
> Cross connect cables between bays does, and between racks does
> not.
>
> Drop wire doesn't. Frame wire doesn't.
I'll believe that it may be the case where you are.
>>I am certainly not in telephony.
>
> Then you should not propose pseudo-authoritative answers to
> questions relating to that field.
Except that we are talking about RS422/RS485 data communications systems
here and not telephony. This is an area that, like Steve and a few others
here, have a fair bit of experience in. I know that some of us are in very
heavily noise ridden environments like myself. I also appreciate that some
of the other contributors to this thread are in no worse than a normal
office environment. So, when it comes to talk of connecting shields I think
we are all well aware of the situations where to connect both ends leads to
problems.
>>> If they had a melted cable, it wasn't simply the difference in
>>> ground potential. (Telecom cables generally are spliced and
>>> grounded every 6000 feet, max.)
>>
>>I know that on navy ships and submarines they connect the outer screen to
>>the feedthrough grounds as they penetrate each bulkhead. There are special
>>feedthrough glands made for this purpose. Then, they also go through a lot
>>of effort to ensure that the electrical ground really is as close to ship
>>ground as possible (usually less than 250mV). A ship or submarine can be
>>considered akin to one building with own power source, however, the cable
>>runs are quite long.
>
> The longest run on an aircraft carrier, or a super tanker, is
> short. More than 3 miles is considered "long" in general,
> though specifically that may not always be the case.
The ships themselves may not be that long but by the time you consider the
routes that some of those cables take to get from one end to the other then
you should not be surprised that there can be single runs that are of the
order of 7km. Then you think about the number of bulkhead feedthroughs and
the mind begins to boggle at the number of clamp-down points that the cable
sheath has to be pared away, ferrules fitted and boots shrunk down.
>>> That is fine within one building, and is not the best practice
>>> for longer cable runs, for reasons explained in the message
>>> listed above.
>>
>>I would hope that it is obvious to all from this discussion that the issue
>>of connecting screens/shields is rather dependent on the circumstances
>>that
>
> You've now mentioned ships and "MEGA-Amp equipment". There are
> many special cases, probably none of which lead to any
> enlightenment except when they are explicitly discussed as
> extremes and flagged as unusual.
In the 30 years of my career it has, to me, been the norm to work with such
environments. My main field of Systems Engineering is Energy and
Transportation. That includes Oil-rigs, Mines, Power Stations (Coal, Oil
and Nuclear), Railway Rolling Stock and Trackside Equipment. I have also
done some work in the Banking Sector and the Medical Devices Sector. I have
to appreciate everything from simple mechanics and low level signal
equipment through to high energy systems. I have also to deal with vacuum
and cryogenic equipment. It has certainly been an interesting career.
> I could also mention a few odd places... the Faraday enclosure
> for radio and carrier equipment at a Coast Guard Loran station.
> Anything near a large power generating facility. The "tempest"
> shielding into a room where the USAF was doing super secret
> seismic analysis (to detect nuclear testing in Siberia). Or the
> operation of electronic in proximity to DEWLINE or Long Range
> Radar sites. For that matter a Toll Center is one of the
> harshest electrical noise environments around.
I am also aware of some of those systems and the issues invvolved.
> These are all wonderful stories and any time you want to swap
> tales, we can. Using them to try to explain grounding on comm
> cables won't help anyone understand what is involved though.
I explained in another reply featuring the diagrams you provided. I am sure
that if you look at the maths of the situations you might get to see the
consequences that most of us here are trying to prevent.
>>the equipment is going into. The screen connection impedance idea that I
>>mentioned in my earlier post would probably work for the widest range of
>>different situations but I would still countenance understanding the
>>issues that surround each individual instalation. It can save a huge
>>amount of grief later.
>
> Understanding the basics comes first. Trying to extrapolate
> these extreme situations to be representative of the basic
> theory is exactly what causes so much confusion.
I maintain that you do the math first then you may see how much and what
effect the various schemes will have. I know that the power levels and
situations that I am dealing with are heavier than most others here but
most of them will also have to understand the fundamental principles
that I have described.
Floyd L. Davidson
Actually, I believe the specification *is* 10 Ohms, and the
target is 5 or lower. An 80 volt difference in ground potential
is... so unusual that we can ignore it. Lets assume it never
gets higher than 20. Or 30, if you like. (Everything I recall
seeing was engineered for 20 V, max.) No doubt there *are*
unusual instances were we might well see figures outside this
range. And if we do, we deal with them as unusual instances...
>I am sure that you can see why we have been advocating the
>grounding at one end only rule as a safety aspectg of RS422/RS485 networks,
>as well as almost any other low level signal measurement/management system.
You are again proposing unrealistic circumstances to portray the
"norm".
>Where you do need to connect the screen wire to both ends then you had
>better include some impedance in the connection that limits dangerous
>current flow and yet provides sufficient coupling to ground for the higher
>frequency signals. As I have stated before, all the circumstances have to
>be well understood to make the right choice.
No. The trick is to ground the cable every 3 or 6 thousand feet, so
there is never get anything like an 80 volt difference.
Can't you come up with something less boorish than repeating the same
thing over and over?
Paul E. Bennett
> http://www.tequipment.net/WilcomTeleTransT124.asp
>
> Here are a couple of specifications for terminating cables
> to building ground systems:
>
> http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
>
http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuidelines/16743%20-%20Telecom%20Risers,%20Closets%20and%20Entries.htm
>
> Here's short URL for that last one,
>
> http://tinyurl.com/b9gu2
>
> Here's an even more authoritative treatment of grounding,
>
> http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc
>
> All of these describe the grounding at *both* ends of a cable.
Just to be clear is what you are refering to as the shield (the bit you
keep insisting must be grounded at both ends and every 3000ft) actually
known to the rest of us as the armouring. If that is the case then it is no
wonder that we seem to have been talking at cross purposes. Yes, armouring
on cables does have to be grounded at both ends. The armour is normally a
very substantial close steel wire wrap whose purpose is mainly mechanical
protection. However, the rest of us, I believe, were speaking of the shield
connection which is a much finer enclosing conductor and which we are all
maintaing should only be connected at one end unless special arrangements
are made.
Steve at fivetrees
news:873brga...@barrow.com...
> An 80 volt difference in ground potential
> is... so unusual that we can ignore it. Lets assume it never
> gets higher than 20. Or 30, if you like. (Everything I recall
> seeing was engineered for 20 V, max.) No doubt there *are*
> unusual instances were we might well see figures outside this
> range. And if we do, we deal with them as unusual instances...
So how do you reconcile even a 20V ground potential difference with the
+/-7V common-mode maximum of RS-485?