PSN-L Email List Message
Subject: Re: Digging in...
From: Gordon Couger gcouger@..........
Date: Sun, 14 Aug 2005 04:12:30 -0500
> In a message dated 13/08/2005, gcc-at-couger.com writes:
> To beat long cable runs and the associated problems
wireless data links
> are just becoming a reasonably priced off the rack
solutions that is
> reasonably easy to put in to practice. Using directional
> and BlueTooth can work several hundred yard in the clear.
> Maxsteam has good off the rack stuff that I know works.
> http://www.maxstream.net/products/index.php If you want
plug and play.
> Hi Gordon,
> Thanks for the feedback.
> I am aware of the radio links that have been / are becoming
> available. If you really need a long link and are happy to
> fine, but do consider all the options.
> UHF radio links do require some skill to implement and
> entirely 'fit and forget'. They may impose limits on the
> computer, both for RAM provision and on the processing rate,
> particularly when using encrypted data. You are usually
> linked applications for seismic data recording. Check that
you can have
> full 'preset' control of the transmitted data rate. This is
> available. (e.g. If I run my 56 K modem at more than 38 K, it
> out' several times an hour / performs like a geriatric snail
- due to
> the length of the line - according to the service provider.)
You need the
> link to work 100% in all weather conditions. High speed UHF
> become garbled in woodland locations, especially during rain
or snow /
> where you get strong reflected multipath signals.
> However, it still leaves you with a remote power supply
> you may need links both ways for seismic work. The link to an
> usually two way. Low noise amplifiers tend to have
> consumption. Do you need to buy battery operated power
> batteries / solar panels? You may then fit an ADC and send
> signals, or you can generate a frequency modulated signals
> demodulated at the receiver, but this may require several
> additional electronics.
> Somewhere along the line you have to have a time
reference on a 24/7
> basis to 1 second, or preferably to 0.1 second, linked to the
> timing. GPS receivers tend to be power hungry, location
> still moderately expensive. The internal software clocks which
> are usually fitted to computers are nowhere near accurate
enough. On my
> 'new' computer, I can't rely on it to within 20 secs per day.
> the clock every hour is just not adequate. Some of the web
> have significant and variable signal delays. (I bought a
> LCD quartz clock.) A 1 sec error on a seismic signal
represents about 10
> km uncertainty.
> Do check on the total $$$ cost / benefit if you are
> radio system. Check if there are similar radio systems
> over similar distances? Are there any adverse local problems,
> radio transmitters / interference sources / obstructing metal
> fences? Try to avoid 'buying trouble'.
> Chris Chapman
I don't think we are to the point that it either easy or cheap
to run out an get a wireless link for home built toys. But it
time to start thinking about them and including them in thoughts
Doing a wireless link is a large undertaking. I did the first
work for Datalink Systems www.rfdata.net wireless stuff and the
power supplies were designed to for 5 volts at 3 amps to
accommodate all the various stuff we supported. However 5V 1A
would meet most installation requirements. They were 30% over
designed hopefully to be longer lived when run in temperatures
beyond their rating. We couldn't get parts to run at the
temperature encountered in some installations in the tropics. so
we were careful to choose chips that had good reputations for
running hot environments and using the highest temperature
rated parts we could find. Using chips like the 68HC11 that
when run at half speed has a reasonable life at 200 c. We
strictly warned against installing things in overly hostile
environments but we did our best to make them live there.
Going wireless some noise problems improve. Using long lever
arms made of light beams can mechanically amplify the signal but
it change the noise problem form electronics to mechanics.
Using microwave particularly the 2.4 GHz band heavy rain and wet
foliage is always going to be a problem. There is probably not a
worse frequency for rain interference in the low GHz band than
they chose for ZigBee. That can be over come with higher gain
antennas and more power. I would use Zigbee because it is
cheap, I know it works and the data format is not set in stone
and it is easy to implement SLIP or something similar that you
can use to encapsulate any kind of format you want with minimum
over head and is easy to modify in the future.
Today I can buy 60 MHz ARM computer board from New Micros off
the rack for $29 bucks high speed SPI or I2c A/D coverers,
memory are cheap and 3o yard ZigBee link that I can be
programmed to use a modification of SLIP
http://www.faqs.org/rfcs/rfc1055.html to implement a limited to
store and forward method for capability for digitized data to
insure data integrity. Made from comments it could be very cost
effective about $100 for the computer and about the same or a
bit less for the solar power. Adding GPS for time somewhere in
the system would cost around $50 bucks. Less for an old Rockwell
Jupiter but that needs to be off the solar power as it draws 200
ma with out a amplified antenna but the receiving station could
use it to send a heart beat time message every second to update
all the clocks on the warless network. If you make if from off
the shelf parts it will run about 500 hundred to a thousand dollars.
Going to UHF radios limits your band width to the IF frequency
of the radio. Of course with ARM computers being a cheap as they
are you can put more compression and computing power at the
remote site than I ever dreamed of.
Embedded computing have changed more in the last 2 or 3 years
than in the past 10 as far and cost and affordability and
increase in computing power and wireless speed that a serious
tinker can buy.
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