Hi All

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.

My problem and questions are related to which network topology and what
protocol I should select:
I have looked at RS-485 and RS422 but I am really not sure at all?

As far as I see, I will only need half duplex so I choose RS485, however I
must be able to poll the devices and return commands as fast as possible.
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.

I looked at the Maxim web site, and with the MAX1486, they promise Data
rates up to 12 Mbit and possible 256 transceivers per bus. I would then need
2 segments to reach my 500 nodes, but that seems acceptable.

My questions are, apart from any comments to the above:

1.   Is RS484 a good choice
2.   What kind of protocol should I select
3.   What about buffering  - Will I need a uart of some sort
4.   Is it possible to achieve data rates of 12 Mbit with a PIC 16F84
5.   What else should I look out for?

Any help in any form would be greatly appreciated

Cheers
Peter Neubert
neubert@bow.intnet.mu