> I am planning a system with a network of PICs.  The PICs will be daisy
> chained together (power and communications) in a standard office
environment
> (fluorescent lights, networked PCs, telephones, HVAC, etc.)  The PICs will
> need to communicate about two bytes (status change of nine LEDs and which
> PIC sent the event) every 5-10 minutes.  Two of the events are pretty
> important and would need to be acknowledged by the other PICs.  Initially,
> the network will be 300' end to end with 18 PICs.  The network could
expand
> to 2000' end to end and about 100-200 PICs.

The CAN protocol is complex, but most of that is done for you in hardware.
You get "free" collision detection, error checking, re-send requests, and
even
a system that will knock a bad node offline if it makes too many mistakes.
Nodes are assigned a priority, so that higher priority ones will seldom wait
for lower priority ones. Speed can be chosen based on max length of net.

If these features sound useful (or required), CAN might be a good choice.
One problem is the expansion to 100-200 PICs. There's plenty of
"address space" with CAN, but the specs for most can transcievers top
out at about 100 nodes per net, so a repeater will be needed to get up
around or above those levels.

The choice of a "Higher Level Protocol" will determine if your CAN net
is "Multi-Master" where every node can transmit at will, or if a polled
system with slaves is used. Many (like myself) just ignore the existing
HLP's and work out what we need on the fly.

CAN messages are 8 bytes or less per message, this sounds OK for
what you described.

So far the 18Fxx8 CAN PIC's are a bit rare, though a few samples have
made it to the light of day. You can still add CAN to any pic with an SPI
port by using the MCP2510. Which ever way you go, you'll still need a
CAN transciever. Most of these transcievers are 8SOP, but I did notice
that Microchip is promising one of their own "real soon now", available
in either SOP or DIP forms.

If you'd like to experiment, there is a pretty cool CAN developers kit
done with PICs at www.diversifiedengineering.net/devel_f.htm .
I built this one based on their documents, and it's been working great
24/7 for a few months now. This is a good way to get started with
proven HW and SW, then edit up from there.

[Shameless Plug] I made mine a bit prettier, let me know if you want
to see a photo. 8^)

A wireless RF PIC CAN node is available from
http://www.autoartisans.com/documents/canrf_prod_announcement.pdf
I have not had the opportunity (yet) to play with this one, but it looks
like a lot of fun! John is a member of this list.

Good Luck,
Lyle
>

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