I like this Q, its got some real meat to it...


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.

No need to use a Lan controller. Besides, the LAN will
run out of gas at about 150meters (timing issues). AND, no
hub is needed.

I've reliably used RS422 at 1100meters at 4800b. You should
use an error-checking protocol; otherwise drop down to 2400baud.

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.

Its easy to bit-bang 4800baud. If the number of sensors is less than 16,
run it at 2400baud. Be sure to use a ceramic resonator for fixing the
frequency; Microchip's badly overstates the reliability of its internal
RC oscillator. Crystals won't hold up if the PCB they are mounted on
must take vibration.

I'd use waterproofed (neoprene coated) CAT5, buried underground at about
3'. It has 4 pairs. You will need a pair for the RS422, then you have 3
more pairs for powering the sensor array. I'd use two pairs (4 wires)
for GND, then +9V on the last pair. You'll need to have a small LDO
voltage regulator for each sensor. Be sure to use MOV's on each sensor
and one on the +9V line. too.

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.

Sounds like fun!

--Bob


Dave King wrote:

> I wasn't quite sure this was the correct topic tag.
>
> I'm working on a project that involves a large sensor
> array spread over a couple acres. I think the longest
> run so far is about 2500 meters (~8000 feet) with a
> sensor every 75 meters (~200 foot).  All the sensors
> are monitored by a master PC.  So the cables have to
> be waterproof etc and neither cause or be affected by
> RF.  Right now the big problem seems to be the layout
> without resorting to Cat-F.  Probably need a few hubs
> buried out there to make it work. Any good reference
> stuff to look at this type of extended net setup?
>
> I've been looking at building the interface using
> either a RTL8019AS or CS8900-IQ. Are there any others
> I should look at?
>
> The actual sensors are pretty simple and a 12C509A
> was selected and easily more than handles (on the bench)
> what it has to do.  I think the 509's will have to be replaced with
> something with a bit more power and pins such as a 870/877
> before it would handle both its sensors and the ethernet overhead.
> Are there any better suited pics?
>
> Dave
>
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