Hi Peter,

   As far as I can see there are two 'very difficult' requirements in your
   request, these are:-
   1. The need to poll 500 devices.

>Temp, Open valve etc). It is possible/probable that as many as 500 devices
>shall be connected to the same PC.
----------<Snip>

   2. The need to minimize response time in an emergency.
   ( you haven't defined what is an acceptable response time.)

>The response time is very important, as the equipment to monitor is very
>valuable and could break down, if proper action is not taken immediately.
----------<Snip>

   These, taken together could amount to a 'deal breaker' I would (politely)
   suggest that you would be very unwise to walk into this potentially litigous
   situation with a home-brewed protocol.
   Translation := 'Go and study the existing protocols like FieldBus etc.'


>1.   Is RS484 a good choice
   No HDX RS485 is better! :-)

>2.   What kind of protocol should I select
   Good question, I doubt that there is an definitive answer, without
   knowing more of the detail.

>3.   What about buffering  - Will I need a uart of some sort
   Yes.

>4.   Is it possible to achieve data rates of 12 Mbit with a PIC 16F84
   This is a crucial question and I suspect that the answer is no.
   I would think a 16c74 as a minimum starting point. With a fair
   effort 1Mbit might be possible, more likely to be less than this.

>5.   What else should I look out for?
   Don't underestimate the difficulty of this task.
   You are potentially exposed to being liable for equipment damage,
   I suggest you get some legal advice. Don't let that scare you off
   however, just make sure you understand exactly what you could be
   getting into.

  On the other hand, IF the question was,
        "Can I network 10-20 PIC systems and remotely control some
        (non-critical) equipment from a PC?"

   then the answers would be somewhat different.

Ray Gardiner, ray@netspace.net.au