Steve,
I've done a bit of research on this. I have a setup that will have in the
order of 70 sensors across 3 buses. My sensors aren't as orderly as you
will be.
As a preface, the 1-wire sensors are cheap, but the communications design
is weak IMO. It is not resilient or noise immune in the way 10baseT
ethernet is. Some people have reported problems with 1-wire and fluorescent
lights, so that's something to think about when running the wires.
First, I decided to go with the ibuttonlink link45 product. My initial test
had 9 sensors and about 700' of cat5e signal run. Things worked fine. I
want to try to run it out to what they say is an approximate limit of 400m
and see how things work. I have found much less published experience that I
had hoped for. Ibuttonlink has both ethernet and USB based products on the
drawing board, but they are a small shop and can only do so many products.
This leaves the serial product for now.
Second, you asked about sensors. I decided to go with the 18B20 in the TO92
package. It can vary between 9 and 12 bits of reading. At 12 bits, I found
them to not quite meet the specs of +/- 1 degree C for accuracy. I don't
know the accuracy needs of the project, but I would plan of +/- 3 degrees C
for the 18S20. In most cases I recommend the B20, because the accuracy is
there and bus time is rarely a problem. In your case, with lots of sensors
on the bus, the S20 may be slightly better.
As for adding power, the ibuttonlink doesn't supply the +5, so you will
have to add a +5 regulated supply between ground and pin 3. I decided on my
own wiring design rather than the RJ45 layout of the ibottonlink product,
so adding this was straight forward. As for your wiring design, I would
give a great deal of thought to that. You will need to be able to add and
drop sensors as configurations change and to be able to diagnose problems.
best of luck,
jerry
Jerry Scharf
laguna way consulting
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