AVR-Chat

> From: Donald H
> ....
> At the tub was an LCD display and a few buttons that show the user what
> was going on, at the equipment room (high voltage) were pumps, heaters,
> blowers, relays for lights.
> 
> ....
> I asked if the two chips were side-by-side on the PCB.
> 
> He proudly says, "there will be 200 feet of wire between them."
> 
> Same problems, same solution.
> 
> Two micro controllers with a rs-485 chip between them.
> ....

I was originally going to use RS-485 but I have decided to use a modified
version of a CAN bus instead.  

The solution you mentioned above is what I eluded to when I said:

"I am looking at using a smaller controller at the "remote" head but this
brings up yet more tradeoff/cost issues and I need to keep it simple at the
moment because there are a host of other issues yet to be addressed in the
system."

The "small controller" I mentioned above is exactly that, a small device
that supports a signal head and shares the same power/data bus that will be
used by the controllers to talk to each other.  The reason I'm currently
driving an LED signal head 800 feet away is for (at least) two reasons:

First: I have not finished designing and testing the communications link
(CAN bus) that will eventually allow all of the components to talk to each
other but this club needed some traffic control immediately.  At the moment
I have two controllers in the field supporting two independent sections of
track.  The ability for the sections to coordinate with each other as well
as other capabilities will be added later.  So, for the moment, the remote
head is driven directly via wire.

Second: The two signal heads attached to the controller ensure that only one
train is allowed into the track monitored by the controller.  To do this, an
approaching train is sensed (before entering the track in question).  If the
controller decides that it may enter, the signal at the far end of the track
MUST go from green to red before (or at the same instant as) the signal for
the entering train goes to green.  This is obviously easy to satisfy if the
signals are directly connected to the same controller.  To satisfy this
requirement over a communications link (with latency, errors, etc.) requires
asynchronous coordination which complicates other things that the controller
is doing, and may even cancel the need for the remote signal to go to red,
hence, I'm avoiding these complications for the moment with wire to the
remote head --- but, I do agree that that is where I need to end up.  Gota
crawl before you can run :-) 

I have about 30 years of experience with different communications methods,
protocols, etc, (Async tty, bisync, SDLC, X25, TCP/IP, custom protocols,
etc., etc.) on mini-computers for applications from financial online
transaction processing to data radio and train control for Union Pacific.
So I've been around this tree before and know where a lot of the potholes
are -- not to say that I might miss one and drop down a black hole for a bit
(no pun intended)  :-)

Now, my experience with microcontrollers, surge protection, PCB board layout
is more limited but I'm learning thanks to lists like this one,
Electronics101, etc.
 
Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck