Peter Neubert wrote:
>
> I need to build (With a PIC) a small device capable of
> monitoring equipment in a factory.
> Each device shall be networked and connected to a central PC.
>
> The PC will then poll every device for information and status of the
> equipment, and if necessary send orders to them (ON/OF, UP, Down,
> Increase Temp, Open valve etc).
> It is possible/probable that as many as 500 devices
> shall be connected to the same PC.

You are re-inventing a wheel.
There are factory automation networks, generically called field buses.
There are far too many of them already, about a dozen.
It is a senseless waste of human life to invent another one.
It takes many man years to develop a decent one.
Take a look at the spec for one and see just how much detail
you need.

I beg you to look around first. If you have spent heaps on a factory,
it makes no sense to pay all the R&D costs of developing a custom
system when adequate ones already exist.

> The equipment to monitor is very valuable So why gamble on the reliability of
a home-made system?
Would you invent your own PC?
In 1984 could have designed a better computer than say the Acorn Atom,
but technology is way beyond that now. Field buses are way ahead
of home-brew systems today. Don't do it!

> SpeedTry Profibus. That's around the 10's of Mbps I think.

> 1.   Is RS484 a good choiceProfibus uses RS485.

> 2.   What kind of protocol should I selectAn existing one.

> 3.   What about buffering  - Will I need a uart of some sortWhat else can cope
 with 12 Mbps traffic?
Siemens have done ASICs for a high-speed fieldbus.
Can't recall which one. Around 5UKP they hoped.

> 4.   Is it possible to achieve data rates of 12 Mbit with a PIC 16F84I'd be ve
ry surprised!

> 5.   What else should I look out for?
> Any help in any form would be greatly appreciated

I used to design field bus hardware.
Even if you get in an off the shelf system, you'll have plenty to do.

For a start, check out http://www.profibus.com/
or Honeywell's SDS (Smart Distributed Sensor) system.