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I'm writing a feasibility report for a project that would involve
placing hundreds, or even thousands, of little nodes consisting of a
really low end PIC, 12f or even 10f, and an PWM'd LED, in trees
outdoors. The project is fairly cost sensitive, given the scale, so I
was thinking of some sort of software one-wire bus implemented on cheap
CAT3. After spending a few hours thinking everything else through, er, I
mean as a responsible first step, I thought I'd try to figure out the
characteristics of my transmission line:

Nodes: 100

Distance needed: ~50ft/node to master, w/ star topology, 5000ft total

CAT3 is 20pf/ft, 30ohms/1000ft

20pf/ft * 5000ft = 100uF
30ohms/1000ft * 50ft = 1.5ohms


Now the line should be acting as a low-pass filter, so from wikipedia:

f = 1/(2piRC) = 1/(2 * 3.14 * 1.5 * 0.0001F) = 1061hz


So I should be able to transmit roughly 100bytes/second?


Yes, I know I'm going to run into lots of other problems... but that can
be saved for the post I nearly sent to piclist... :)

- -- 
http://petertodd.org
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