Multiple AVR Micro Communications , Need comments

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Hi,

    I want to set up 3 AVR micros, one master and two slaves. One of the slaves
is 750 feet from the master and the second slave is 300 feet away from the
master in the opposite direction.  The two slaves will provide sensor input to
the master which will then tell the slaves to turn  motor relays and valves on
and off. This is an irrigation supply (slave-1) and holding pond (slave-2) and
the master (in between).
    I am looking for something simple, low cost and reliable. I have worked with
microcontrollers for a while but have not done anything with communications
between micros.
    This doesn't have to be hi speed since most of the information will be in
bits.
    I am looking for comments and/or suggestions.

    Thank you,
    Andy in Yakima

Re: Multiple AVR Micro Communications , Need comments
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RS-422 / RS-485 serial should work for that speed/distance.  With a
master/slave config, they can all be on the same serial "bus" without
collision problems (avoidance isn't hard).  If you can get a PPP stack to
run over it, it'd be nice for development and future flexibility, but even
a simple MODIS protocol can be changed to MODIS over TCP/IP later on.

--
Ron Sharp.



Re: Multiple AVR Micro Communications , Need comments
On Mon, 15 Sep 2003 13:46:05 -0400, "Android Cat"

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RS-422/485 is a good alternative for this kind of system.

However, with the devices apparently in the field (not within the same
building), you might experience ground potential differences between
the sites (due to voltage drops in the neutral mains wire) which might
exceed the common mode voltage range (often -7 to +12 V) of the RS-485
transceiver chips. Thus, you might have to use optoisolated
transceivers.

With RS-485, remember to turn off the transmitter when the last bit
has actually been sent, _not_ when the last character is loaded into
the UART.

With low speeds (9600bit/s and below) you might also consider the 20
mA current loop, which is easy to opto-isolate with two optoisolators
at each station.  You need a floating 20 mA constant current source
(powered from at least 24 V) and the receiver (the LED in the receiver
optoisolator) and the transmitter (the photo transistor in the
transmit optoisolator) are connected in series and all stations are
connected in series into the loop. Thus you need a single pair to each
slave. The receiving program should ignore the characters transmitted
by the same station.

Wires stretching out in opposite direction form a quite effective
antenna, thus, any lightnings might induce quite large currents into
the wire, so it is a good idea to keep the cable optoisolated from the
rest of the system, even if the ground potential differences in a
normal situation would not require it.

Paul
  

Re: Multiple AVR Micro Communications , Need comments
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Optoisolation is always a good idea in the Real World.

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Depending on the serial 'chip', that can be tricky.  As I recall, some
will show/interrupt buffer empty while the last byte is still being
shifted out--It's a good idea to read the specs.  There's usually a
seperate status flag that the shifter is empty too, but don't check it
until the last character.  Hmm, and allow time for the transmitter to turn
on/off, but that might have been the serial chip used.

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Of course, if there's a break anywhere in the loop, everything is out of
touch.  However it does make detecting communications failure easy.  (The
sound of an open-loop ASR-33 TTY is such a classic!)

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If it is a factory environment, occasional arc-welders and spikes from
heavy equipment.  Video arcades were nasty power environments back when.
I think of optoisolation as a fire-wall for real fire.  (And the sound of
every chip suddenly cratering and letting out the magic smoke often
offends. :^)

--
Ron Sharp.
; Output character direct to port, switch on 485 drivers.
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Re: Multiple AVR Micro Communications , Need comments

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I agree.  Look up the SN75176 or MAX487 - nice RS485 driver/receivers
that are easy to use.  Either of these can interface to your onboard
UART, use another pin to control the tx enable / rx enable lines of the
driver/reciever.  Thus, the only difference to your software from using
RS232 is that you only need to enable the transmitter when you are
getting ready to send data, and then disable the transmitter when you are
done.

Cheers,
-Brian
--
Brian Dean, snipped-for-privacy@bdmicro.com
BDMICRO - Maker of the MAVRIC ATmega128 Dev Board
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Re: Multiple AVR Micro Communications , Need comments

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slaves
input to
valves on
and
with
communications

Looks like an excellent application for ISM band RF.
YOu may want to check out the Atmel AT86RF211 and similar devices
http://www.atmel.com/products/SmartRF/

Ulf Samuelsson
Atmel



Re: Multiple AVR Micro Communications , Need comments
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If only they weren't so damn expensive... *wistful sigh*

Re: Multiple AVR Micro Communications , Need comments

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What expensive, naah, I get them for free..., but You already knew that
Lewin :-)

Seriously, How much does it cost to draw that cable?
I digged down 300 meter cables during the summer for a nice Electrolux
Automower.
Now I am happily watching it mow the lawn, but at that time,
I can assure you that I gladly would have payed $100 to avoid digging for
the cables...

--
Best Regards,
Ulf Samuelsson   ulf@a-t-m-e-l.com
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Re: Multiple AVR Micro Communications , Need comments
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:)

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Actually I was thinking about them for *my* project, which requires
wireless.

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It's good practice for the ditch-digging jobs we'll all have in ten
years, when the only OSes and applications that are legally saleable
require cryptographic DRM hardware and all the development work is
done in China and India.

Re: Multiple AVR Micro Communications , Need comments
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 Thank you all for the response!
I think I will start with RS485. This is irrigation country so we are use to
digging ditches, although I am from the school that digs shallow ditches. I'll
stick my twisted pair cable in PVC pipe just below the surface.

    I've thought of using many ideas; RS485, RF, opto using long range LEDs, and
even DTMF tones.

I have a portable phone( NOT cellular) that I use on this property. The base is
at the master site and it looses reception at the canal site so if I used RF I
would need to install an outside antenna at the master site.

The long range LEDs and DTMF tones would need line of site unless I could
incorporate them in the cable and RS485 seems the simplest over the cable.

I have down loaded National's application note Ten Ways to Bullet Proof RS485.
I've also down loaded Atmel's UART code, and avrfreaks has note on RS485 timing
issues.
Is there another application note or web site that I should take a look at?

Thanks,
Andy in Yakima

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