Is termination needed for short and slow RS485?

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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.

Are RS485 terminators really needed for this?  I know they are needed
for 'long' cables and 'fast' baud rates, but does 50 feet at 2400 baud
qualify for this?

And yes the reason I am asking is there are some problems with some of
the displays.  (If it was easy to just plug in some terminators at an
existing customer site with the problem I would; but it's not easy.)

Also, it may be that the cable being used it not twisted pair.  Is
that important for this setup?

Thanks very much.
snipped-for-privacy@yahoo.com

Re: Is termination needed for short and slow RS485?

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Probably not. However, you may still need pull-up/pull-down resistors.

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If total cable length is less than, say 1/16 of a bit time, you
probably don't need terminating resistors.

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Depends on how noisy the environment is.

--
Grant Edwards                   grante             Yow!  We just joined the
                                  at               civil hair patrol!
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Re: Is termination needed for short and slow RS485?

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Just to give an idea of how noisy some environments can be:
At a shopping center with cable runs up to 500m, we have picked
up noise spikes over over 200V that lasted in the ms range. This was
using good quality twisted pair cable with a defined impedance.  This
was measured directly over a 120 ohm termination resistor.  


Regards
   Anton Erasmus

 



Re: Is termination needed for short and slow RS485?
On Sun, 19 Sep 2004 22:33:17 +0200, the renowned Anton Erasmus

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Galvanic isolation of the bus may be a good idea.

Best regards,
Spehro Pefhany
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Re: Is termination needed for short and slow RS485?
I read in sci.electronics.design that Anton Erasmus
eranews>) about 'Is termination needed for short and slow RS485?', on
Sun, 19 Sep 2004:

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It can happen, but you have to be extremely careful how you do the
measurement. 'Balance' in balanced circuits means 'balanced impedances';
it has nothing to do with signal voltages. The line remains balanced (or
not) when no signal is present.

So, you need to make sure that the sending-end impedances from each
conductor to ground are closely equal and preferably low. At the
receiving end, the impedances to ground must also be closely equal, but
in the interests of signal transfer, especially at low frequencies, are
preferably not equally low. This is because the four impedances form a
balanced bridge and the error voltage due to the impedances not being
exactly equal, which is differential and becomes an inextricable part of
the signal if it's in-band, is smaller if the impedances at the sending
and receiving end are very different.

'Closely equal' can mean really close. To get 80 dB common-mode
rejection, the impedances have to match within 0.01%.

At higher frequencies, of course, the *differential* impedances at each
end may well need to match the line impedance.

Measuring such 'highly balanced' circuits requires extreme care,
otherwise the measuring instruments upset the balance and produce
seriously pessimistic results.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
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Re: Is termination needed for short and slow RS485?

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Wow! John, that's good info I haven't seen before. Thanks!

John




Re: Is termination needed for short and slow RS485?
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It could work without teminating resistors.
What is essential is the common more range of some
7 V or so. So you should always have the GND as
reference together with the differential signal.
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. (...)

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: Is termination needed for short and slow RS485?

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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.
 
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This is essential.

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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.  
 
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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.

Paul


Re: Is termination needed for short and slow RS485?
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 >
 > [snip]

You're perfectly right Paul.
I was somewhat unclearly indicating that the GND
should be designed into the system, into the connector
and into the cable. Current flow through it should
be avoided. Should at some point be decided that isolators
were required, the GND could be used. The 100 Ohms in
series with the GND could als be employed to detect
current flow. Depending on the location, measuring
GND differences are unpractical.
I actually run +5V together with the GND too. This was
isolators can be fully powered through the cable.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: Is termination needed for short and slow RS485?

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I suggest you start with this article written for Circuit Cellar by Jan
Axelson...

http://www.embeddedsys.com/subpages/resources/images/documents/microsys_art_RS485.pdf

Bias the signal leads per Jan's recommendations. Also note Rene's comment on
common mode voltage and the fact that three wires (not two) are required
(even if one of them is the DC ground wire).

You'll probably have to provide terminations on one of the signs that is
giving you problems, just to see if it fixes the problem.

I designed the electronics hardware and wrote the software for the full-size
and mini message boards shown here...

http://www.precisionsolarcontrols.com /

The balanced data bus is RS-485 at 115,200 bps using randomly-dressed
non-twisted 18-gauge wire. There have been no problems. I don't recommend
such haphazard wiring without extensive testing.

Good luck.

John



Re: Is termination needed for short and slow RS485?
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 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.

Re: Is termination needed for short and slow RS485?
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That 100 ohms could function as a fuse.  Grounds can have massive
differences, think start-up currents into elevator motors and the
ilk.  The fault there is the connection to 0v, assuming that is
connected to local ground.  Any long wire can have large voltages
inductively induced by nearby equipment.

--
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   Available for consulting/temporary embedded and systems.
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