Home automation idea, RS-485 over cat5 lines in home?

Recently I started thinking about ways I could automate/log sensors in my home at the lowest cost possible (aka, not ripping apart walls to install wiring, and not forking out heaps of cash for ethernet shields). I really like the idea of RS-485 in general, mostly due to the simplicity and cost of transceiver chips (under $2 a piece). The general idea would be to place a 'node' in each room, which would consist of an Arduino/microcontroller and a RS-285 transceiver chip. Each node would act as a slave device and would communicate with a centrally located master. Initially I considered connecting all of the devices wirelessly via radio, which I suppose would be fairly cheap, but I prefer hard wires connections. So it got me thinking, why not use an existing CAT5 plug found in most rooms? I believe there's typically a couple of unused twisted pairs.

So basically I'm wondering how feasible this idea would actually be. Some potential problems I'm looking at: 1) With RS-485, nodes should be placed in a line-like fashion, which isn't exactly possible with the existing ethernet configuration, as the lines propagate outward from a central switch/router. There could be some impedance/resistance issues. 2) Data could be effected when passing through switch/router (which I could bypass externally from router)

Anyone have some insight on this? I'm not 100% set on RS-485 and the CAT5 approach, other ideas are most welcome. Other thought would be to transmit signals over power (bit risky, not totally comfortable with this), transmission over phone lines (illegal I'm assuming), data via ethernet (ethernet chips are pricey), or focus on wireless via radio.


Ethernet is in a nutshell a over glorified serial connection, and if your running <1GBs there are unused lines

I seem to recall reading that star topologies are possible with RS-485, and there are some hardware concentrators that terminate each line separately, although it might not be necessary in a home.

Actually I just found this, maybe some light bedtime reading? LOL... http://www.ti.com/lit/an/snla042a/snla042a.pdf Page 4 has some info on star topologies with RS-485.

Starr topology is definitely frowned upon with RS485, however if the speeds are low I would think it will work. Alternatively have a transceiver per line to make it X point-to-point links and to the "staring" with a mux or other logic.


Hmmm, yup I figured there would be an issue with the star configuration. I may end up going for a wireless solution, probably rf, mostly as they are quite inexpensive. Will have to think a bit more about it. Thanks for the link spirilis, it's a good read!

Thanks for the help guys.