Wire communication on long distance (>20m)


I just moved in a new apartment and there are alarm cables wiring in every room.
I want to take benefit of that wire network and build my own sensors (Arduino Pro Mini + PIR Sensor + Humidity + Temperature)
The central computer gathering data+web browser is a Raspberry Pi (or BeagleBone Black if I need more cpu power)

The alarm cable is 6x0.22mm wires. (http://image.made-in-china.com/43f34j00aKEQsWCBbVce/Shielded-Unshielded-Alarm-Cable-22AWG-2c-10c-0-22mm2-Tc-Bc-CCA-CCS.jpg)
There are in total 3 cables loops with at max 4 sensors on a loop.

| central board |
   |   |   |
   |   |   +-- sensor
   |   |
   |   +-- sensor 1 -- sensor 2 -- sensor 3 --sensor 4
   +-- sensor 1 -- sensor 2 -- sensor 3

I'll connect 2 wires to a +12VDC 1A power supply, so I'll have 4 wires left per loop.

Problem: the farthest sensor is >20meters away from the Rasberry Pi central unit. I doubt I2C or RS* protocol will work on long distance with such low quality cables.

What are my options to use the wires in place and not use RF signals ?




I found out that in fact, I could use RS485.
I'm not sure about the wiring. Cheap arduino compatible boards on ebay (http://www.ebay.fr/itm/170934217208) have only two wires. I've read several topics about a 3 wires version of it (one wires for A to B, one for B to A, the third one for the ground)

Any feedback on RS485 with several arduino ?

I have the same question. But nobody answers!

Same question here but with an addition. I need to power the Arduino from the central one so also wondering for a solution combined with RS485. Would be perfect to use XLR cables/connectors but not sure if that's possible. I am afraid that I need 4 wires instead of 3 but maybe some shielding for the ground connection?

Do you have 6 core alarm cable?