>reliability on 2 wire

You can always use a non-zero voltage as 'LOW' f.ex. by using a zener to
pull down the line to transmit, and have the power supply be a low current
supply above zener voltage and a higher current supply below. Then add
chip fuses in each handheld box and wire branch box. If something shorts
the wire or if a box is destroyed or whatnot then the respective fuses
will go. You can help the fuses to go by putting a crude humidity sensor
on the PCB (interlaced traces) to drive an extra (low) power transistor
that shorts the input wires and helps the fuse to depart. I have used this
method and it works (in another project where things tended to get wet and
then go berserk and interfere with signalling). I also used little 5mA
LEDs that were in series with the line in each button box and a bridge
rectifier in each box to avoid polarity problems. This was not a PIC
project, it used 4XXX CMOS ;-). I also had three steps of current from the
PSU so the user would get feedback on the LED when pressing a button.

hope this helps,

Peter

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