> Hi,
> I have been looking at using the Pic micro as the base for a home / building a
utomation system.
> I have worked out designs for a one wire pair system (caring power & data ) an
d a two wire pair system  (separate power & data pairs).
> I have not used RS485 in my design because I wanted to use a I2C style of BUS
ARBITRATION on a differential twisted pair wire.

You can use I2C style arbitration on differential twisted pair.
CANbus does it.  The trick is to drive the RS-485 diver enable
signal with the serial Tx line.  Properly done, the driver circuit will
work transparently on any RS-485 network.

See National's Application Note AN-915 regarding the SAE J1708
spec.  http://www.national.com/an/AN/AN-915.pdf

AN-915 makes the case for National's DS36277, since it eliminates
an inverter.  You can still use a 75176 if you use a 2N2222 to invert
the UART output, or if you are bit banging, invert the signal in the
software.

regards
Thomas J Macauley, KD7BDW
thomas@advancedcontrol.com
(208) 362-5858