Wirng considerations of a 1-wire network

[Michael McSharry]

I ran across a Dalsemi app note on the design of the 1-wire network. It provides a good discussion on the wiring considerations of the network. In particular, if the bus is long, then inclusion of the 5v power line will degrade performance because it introduces additional capacitance. Similarly the other wires in the bundle should not be terminated, but left floating. Using the same bundle for multiple purposes also has the same degrading effect.

1-wire Net Design Guide

[donstephens]

I read (twice) this document, but it's over my head. I am trying to design a one wire system for a out building that is a bit over 350' away from the monitoring computer. Reading the above mentioned document, it appears that the best wiring structure is cat5. Is this what most people are using? This is new construction with the walls open (as is the house, at this point) so I'd like to prewire for some temp, light, and possibly humidity sensors. If someone has done something similar, I could use some hand holding.

Thanks

Don

[Michael McSharry]

My take from the design guide in your case is to use low capacitance cabling because of your long run. I have never looked at the spec's on the CAT5 that I purchased at The Home Depot, but I did measure waveform rise times on that CAT5 length similiar to what you are needng and found it to be about 20 microseconds which is too much for reliable operation.

In another reference I found the recommendation to use two wires that are not intentionally twisted. (e.g. two solid color in a CAT5 bundle) to reduce capacitance. I did this in my setup, but never remeasured the waveforms. I suspect the trade-off is less immunity to noise. If noise introduction is a problem then a shield in the case will work. When a shield is used then connect only one end to ground and leave the other end unconected.

Running a third wire for power will increase capacitance as well. Unused wires should be left open at both ends. As a minimum this means that you should not try to use wires in the the 1-wire bundle for other applications because those will introduce capacitance and degrade the 1-wire.

My experience indicates that there is some art to this. I tried to improve my wiring architecture to better conform to the recommendations and my reliability decreased. I do have a an extra set of homerun coax cables that I wired when I built my house that go to every room in my house. Seems like these should work well for 1-wire, but I never tried, nor do I have cable-engineering background to understand why I should or should not use them for something like this.

[donstephens]

I'll look into this further...