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http://groups.google.com/group/comp.arch.embedded/browse_frm/thread/fb42bb6a65b34a55/46311396559cd166?lnk=gst&q=RS485&rnum=24#46311396559cd166

earth loop problem Edit

{{{

n article <9bce2282.0409190911.119e2...@posting.google.com>,
   mrsoft99 <mrsof...@yahoo.com> wrote:

> We are running some led billboard displays using a multi-drop
> 2-wire RS485 network. The cable length is 50 feet; the baud rate
> is 2400 baud.

 It is better to use 3-wire for differential comms
 so that the receivers have at least some idea of
 the common mode voltage. Also a low-pass filter
 on the input of each receiver can be useful in
 avoiding difficulties with line-ringings, (from
 a line that is being driven too hard), or stray
 interference.

 RS485 lines.
 A  B  0V
 |  |  |
 |  |  |    2k           _
 +--|--|----/\/\---+----| \
 |  |  |           |    |  \
 |  |  |    2.2nF ===   |Rx >--
 |  |  |           |    |  /
 |  +--|----/\/\---+----|_/|
 |  |  |    2k             |
 |  |  |                   |
 |  |  +----/\/\-----------+---0v
 |  |  |    100R
\|/ | \|/
 More receivers, as above.

 2400 bits/sec is 417uS per bit.

 2+2k*2.2nF is an 8.8uS RC time constant, which
 will get to 99.9% of where it is supposed to go
 in about 44uS, or about 10% of each bit-width.

 The 100R resistor connecting each receiver 0v to
 the line 0V is there to limit the current in case
 of earth loops.  In fact if there is any voltage
 across any 100R (when not transmitting) then you
 know that you probably have an earth loop problem.

-- 
Tony Williams.

}}}


Paul Keinnan earth signal wire Edit

{{{

On Sun, 19 Sep 2004 20:45:28 +0200, Rene Tschaggelar <n...@none.net>
wrote:

>mrsoft99 wrote:
>> We are running some led billboard displays using a multi-drop 2-wire
>> RS485 network.
>> The cable length is 50 feet; the baud rate is 2400 baud.

At least you should have somewhere a pull-up and a pull-down resistor
to drive the bus into the idle mark ("1") state when there is no
active transmitters.

>It could work without teminating resistors.
>What is essential is the common more range of some
>7 V or so.

This is essential.

>So you should always have the GND as
>reference together with the differential signal.

This might generate more problems than it solves.

If the equipment are ground referenced to the electric system neutral
(directly or through PE connected to neutral), the reason for the
equipment ground potential differences is the different voltage drops
in the mains system neutral wiring carrying the whole mains neutral
current. The drops can be quite with large single phase loads or even
in a three phase system there can be quite considerable 3rd harmonic
currents at 150 or 180 Hz if there are a lot of switching mode power
supplies in each phase.

Connecting the signal ground in the signal cable between two equipment
that also are referenced to the mains neutral will effectively connect
two points in the mains neutral together. If these two points in the
mains neutral network have a different potential (due to different
voltage drops), some of the neutral wire load current will flow
through the signal ground in the signal cable between the two neutral
connections. This current could be several amperes and in the worst
case even burn some PCB ground tracks. This current could be mainly at
the fundamental 50/60 Hz frequency, but at least also in three phase
electric distribution systems at 150/180 Hz and in addition all kinds
of high frequency interference from switchers etc.

If the distance between the two equipment is the same measured on the
serial cable and along the electric network, the ratio between the
current flowing in the neutral wire to the current in the signal
ground is directly proportional to the cross section area ratio
between the mains neutral and signal ground.

A thin signal ground between the equipment does not carry a lot of
current, so it does not reduce a lot the ground potential difference
(and hence common mode voltage) between the equipment. If a thick
serial cable shield (with cross section comparable to the mains
neutral cross section area) is used, about half of the mains neutral
current will flow through this shield and drop the voltage difference
between the signal grounds to one half of the original value. However,
if such system is used, it is essential, that the shield is connected
directly to the equipment chassis and then to the mains PE and not let
this large current flow through the PCB of the equipment.

If the worst case ground potential difference is non-zero but well
within the common mode range, it should be sufficient to run the
connection without a separate signal ground and rely on the mains
grounding.  

>If there is the chance to exceed the common more range,
>couplers should be inserted and held at the GND
>potential. These 7V may not be much and are quickly
>exceeded, and be it only at burst noise.
>It happened in one building with an inhouse-net,
>that communication failed when the lift was running
>even though the RS485 was held at EARTH potential.
>Yes, the lift made the EARTH jump by more than
>the common more range. (...)

Galvanic isolation from the local mains grounds is required, if such
large variations exists, since a thin signal ground would have no
effect on the voltage difference in the mains neutral wiring.

IMO, a separate signal ground in the RS-422/485 cable should only be
used, if all the equipment are floating relative to the mains
grounding. In this situation, it simply is supplying the bias currents
for the receiver input transistors.

With floating system, the signal ground wire can be eliminated, if the
transistor bias current is supplied some other way.

On RS-422 connections, the "fail-safe" termination (a voltage divider
between local Vcc, the signal Rx wires and the local signal ground)
should supply the bias current. On multidrop RS-485 the A and B signal
lines should be terminated at both ends of the bus as usually with
resistors, however, high resistance (1-10 kohm) pull-up/down resistors
from _local_ Vcc and ground to the signal line are required to supply
the bias current for each transceiver. The internal power supplies
within each station will be floated to approximately the same
potential without any external ground connections.

}}}

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