Dave King wrote:

>
> I didn't think 422/485 would handle the long runs. That sure would
> make life easy if it could work with it.

It can easily handle a one mile run. I've personally tested it with 4
1000' rolls of spliced CAT5 at 4800baud. You'll need to use maxim's
low-power tranceivers to put 250 on a single cable without a repeater.

>
>> 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.

All the more reason to use RS422/RS485. The overhead WOULD choke up a 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.

You'd have an enormous mess with TCP/IP, and each sensor would be bulky,
use too much current, and would need lots of $$ to get it going.


> 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.

All the more reason to shield and bury it. Even RS422 will generate RF
on sharp data edges, although at 2400baud, the capacitance of the cable
should soften the edges greatly.

> Actually the more I look into this I think a nice paper route is the way to
> go....be my own
> boss, flexible hours...;-]

I did that as a kid. You wouldn't like it.. its a thankless job.. like
being an engineer all these years.

--Bob
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                Bob Axtell
        PIC Hardware & Firmware Dev
          http://beam.to/baxtell
              1-520-219-2363

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