PSU/RS485 Problem, cant make my mind up.

I am designing a RS485 remote control system, to switch relay outputs
on some 32 terminals, over an area of some 250 Metres by 200 Metres. I
have got a lot of information from the DCI Bus system from Elektor
Magazine (December 2002), the only real difference is I am using a PIC
Chip with a USART and Max 487e chips for the RS485 Drivers/Receivers.
I was intending to use a 3 wire system with two for the RS485 and one
for a common ground via a 100 ohm resistor to each board.

The problem is, how do I power the 32+ terminals. I could if I have
to, insist each terminal has a mains power supply available, so power
supply for each individual terminal was not a problem. But this would
be problematical, it would be easier if only every 6th or so, terminal
had a power supply, and all the terminals shared their power with
adjacent terminals, via a pair of twisted cables I have spare, over
from the RS485 system. I would fuse each terminal, so no one terminal
could short circuit the whole system Each terminal would be fed from a
1N4001, so that volatge differences in the power supplies did not
cause a problem. But what I cannot work out is the grounding. In the
DCI bus I mentioned earlier, the Signal grounds were via a 100 Ohm
resistor to signal ground. But if I have a common power supply
arragement, then I think I cannot use the 100 Ohm "decoupler". I need
any thoughts you have on my proposal, thanking you in anticipation.
Mark in Spain
mark@xxxxxxxxxxxxxxxxx (remove the X to reply)
.

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