Sending I2C reliabily over Cat5 cables

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Sending I2C reliabily over Cat5 cables

Postby niko » Fri Dec 09, 2016 8:26 am

I am considering implmenting an home automation system around my Raspberry Pi but I found the price and space requirement of inserting a Pi every place some control is required too much but the Cat5e cables required for this design is already installed during renovation. I have some PCF8574's, PCF8591's and SSRs lying around so is it possible to drive them using Cat5e cables?

All my Cat5e cables are already wired with TIA/EIA 568B pinout. They are part of my structural cabling and are not shielded, so higher line voltage is required. I am thinking sending power and I2C lines over the cable, with this pinout:
Pin 1 (Pair 1): SCL+
Pin 2 (Pair 1): SCL-
Pin 3 (Pair 2): SDA+
Pin 4 (Pair 3): +12V
Pin 5 (Pair 3): +12V
Pin 6 (Pair 2): SDA-
Pin 7 (Pair 4): GND
Pin 8 (Pair 4): GND

The power pin arrangement is same as 100BASE-TX PoE wiring so power rating will be the same too, and using of bidirectional differential signaling is found in 1000BASE-T which requires Cat5e.

Original I2C SCL and SDA lines are derived into two bidirectional differential pairs at TTL levels (the open-drain is not kept on the wire, but restored in the line termination/level shifting device that I am designing)

Any suggestion on that? Also, which chip should I use to convert I2C lines to the differential signalling? Please suggest chips with DIP through-hole option to me. I do not know how to handle SMT stuff.
I found this chip, SN65LBC180, is it a good choice? How to wire it into a bidirectional unit? How to shift level (it is a BiCMOS part requiring TTL level but Pi drives at 3.3v CMOS levels) and make it open-drain-compatible?
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