>Given that I need to have address discovery, then yes, 2 way
>comms has to be possible. I might want to put sensors in the
>network too later on. Being able to upgrade the firmware of
>the devices via bootloader would be really cool too, except
>that no 12F or 10F part supports it.

Not sure if the small 16F parts support it. Would also give you enough pins 
to drive multiple colours of LEDs at a node.

>Now by current modulation are you assuming that the nodes
>will work as some sort of current loop?

No

>My understanding is that I'd be doing voltage modulation
>for both ways. The slaves would short Vcc/Data and GND to
>indicate zero's. Given, say, a 5k pull up and a short through
>a 200ohm resistor the slaves would be pulling the line
>down to 0.2V

OK, that is a similar system to what I am suggesting except you are clamping 
the voltage right down close to 0, which means you can use voltage sensing 
at the power supply end. However this will mean that you will need to have a 
power supply that has a sharp knee on its current limit, possibly even some 
form of foldback current limit, and the pass element has the possibility of 
getting very hot - remember this will have the whole power supply input 
voltage across it while the node is shorting to ground, ant will need to be 
current limited at a touch over 2A for 100 nodes at 20mA/node - >2A if you 
want expandability. Not only that but the transistor at the node will have 
to have this current capability as well. Note that unless you have some 
large capacitance at each node, isolated from the supply by a diode, you may 
see the transmission modulate the light level of any LEDs that are turned on 
due to the chopping of the supply voltage.

The current modulation scheme is similar at the node end, except it uses a 
higher value resistor, so the node can turn on sufficient current to be able 
to be seen at the power supply as a current fluctuation. Not only that but 
remember you have other current fluctuations caused by LEDs turning off and 
on.

...

>I'm going to leave the checksum off as integrity isn't critical.
>This is just a light show. :)

But you will want a checksum if you start doing firmware downloads. 
Depending what you are using the LED display for, you may still want a 
checksum of some sort, there is nothing more frustrating than having such a 
system go AWOL in sequence, even if it is only a demonstration Christmas 
tree lighting display.

>Sounds like a very robust system, but I really don't think it's
>doable cost wise. For even just two pairs the wiring cost, 50ft
>per node, will end up at $2.50 per node assuming I buy spools of
>4 pair CAT3 at $200 per 1000ft.

...

>Another problem with full cat 3 is visuals... it's big and ugly.
>If anything I'll may very well have to find a source of jet black
>twisted pair somewhere, skinny and unobtrusive will be important
>during the day.

I would see if I could find some dark green sheath CAT wire - like used on 
Christmas tree lights. As to the component cost, I would try hitting up some 
FAEs for chips - I am sure you could get a couple hundred CAN drivers & PIC 
chips for 'demonstration purposes' especially if you are prepared to write 
an app note of the experience (student task - part of the project!). You 
might even get the same consideration for a roll of CAT cable with a special 
coloured sheath - would the army use CAT5 with camouflage green sheath? With 
the right colour I think you would find that even CAT 5 would get reasonably 
invisible.

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