If you really want to go the wired way, be aware that 10Base-T  (10Mbps
ethernet over cat5) is typically rated for only 100 meters, with up to
four repeaters/hubs per run.  This means you could have a maximum run of
500meters.

RS-485 is better suited for your job.  At 9.6kbps a single run without
repeaters can go 1200 meters.  You can use 9 repeaters per run, and have
a total bus length of 12km.  The bus would only support 32 devices at
this length, though.

At slower speeds and shorter lengths with better transceivers you can
get 256 devices on one  very long segment.

The RS-485/422 specification only covers lengths up to 4000 feet, but
newer devices can reach well past that, and repeaters consist of single
chip solutions.

Lastly, ethernet devices built today aren't made for outdoor use and
extremes associated with that environment.  It'll get very expensive.

The article where I got the length limitaitons of current rs-485 devices
is here:
http://www.phoenixcon.com/info/articles/copper_passemain.shtml

Good luck with your project!

-Adam

Dave King wrote:

>> I have done several long-running arrays (casinos).
>>
>> The easiest way to do this is to use RS422 drops at all
>> the sensors and at the PC, and run the whole thing at 4800baud.
>
>
> I didn't think 422/485 would handle the long runs. That sure would
> make life easy if it could work with it.
>
>> Your 12C509's will work fine, but you'll need an RS422 transceiver chip
>> on each sensor. With that much cable, you should use bidirectional MOVs
>> on every PCB to protect from lightning and ESD.
>
>
> The actual data I'm dealing with is very limited. For the most part it
> consists
> of exchanging a few bytes of data every few minutes. My concern was that
> the overhead from using ethernet would choke the pic.
>
> Part of the reason for considering tcp-ip/ethernet was the number of
> required
> sensors  On this demo sized field I'd guess I need to talk to around
> 250 nodes.
> On a large field they would deploy anywhere from 3,000 to over 10,000
> nodes.
>
>> PS Its easier to bury stuff underground that you realise. You can rent a
>> vehicle with a funny-looking tail that can dig a 3'deep hole 2" wide,
>> and you can walk behind and drop the cable it in as you go along.
>> Probably get it done for $100 USD or less.
>
>
> This is all underground wiring. Even on the small sensor matrix I think I
> figured on
> about 2.5 miles of trenching for the basic wires.
>
> I had considered RF but they do not want any emissions on any
> frequency. The
> demo is planned to be setup near a radio telescope so I guess they
> want to
> be good
> neighbors.
>
>> Sounds like fun!
>
> Actually the more I look into this I think a nice paper route is the
> way to
> go....be my own
> boss, flexible hours...;-]
>
> Now I'm sitting around here trying to think what kind of sensor you
> use for
> dinosaurs or bigfoot...
> That and if Russel or Jinx ever stop posting I'd actually get some work
> done ;-]
>
> Cheers
>
> Dave
>
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