AVR-Chat

> From: Mike Payson
> ....
> 
> I'm confused by the power issue... You say you are already providing
> 48vdc, so why not just step the power down for the Zigbee? Running
> cable isn't quite as bad since you already have the conduit, but it
> still seems that running cable would be a hassle, and the cost would
> be almost identical or even cheaper.

No problem powering the Zigbees from 48v.  I'm already planning on
downconverting the power with a switcher regulator to power the nodes (the
reason for a 48vdc bus rather than ac, etc. is a whole other discussion that
I won't go into here).

I only mentioned the 48v power cable to say that there is already a conduit
to all the (current) node locations.  The point being that it is minimal
effort to pull the CAT-5 with the power cable and the only added cost is the
CAT-5 itself which was costing us ~$55 per 1,000 feet.

From Wikipedia: "Transmission range is between 10 and 75 meters (33 and 246
feet) and up to 1500 meters for zigbee pro, although it is heavily dependent
on the particular environment."

I see on the Sparkfun site that 1mw units are about $23, 2mw $26-$29, pro
$38-$45.

(what follows is of the top of my head ...)

So, assuming two nodes separated by 1,000 feet, it costs me about $28 per
node with CAT-5, and (since I have to use Pro), let's say $40 per node -
and, that's assuming that Pro can reach that far in a heavily wooded area.  

To be fair, since Zigbee can communicate in both directions (east/west on a
track for instance), let's say 5 nodes, each separated by 1,000 feet, that's
$220 for cable (4,000 feet @$55/1000) or $44/node.  For Zigbee Pro, that's
$200 (5 units @$40) or (obviously) $40 per node.  

Sounds about the same, but: I would have to test to confirm that Pro can
cover the distance in a heavily wooded area - I know cable works reliably
that far.  If a Zigbee link doesn't work someone has to diagnose if it's an
RF problem or not and then track down the source requiring somewhat special
knowledge and equipment - a cable is a comparatively easy thing to diagnose
with minimal tools, and knowledge.

Now, suppose the nodes are clustered in a smaller area (higher density), my
cable costs per node go down dramatically but the Zigbee costs can (from
above) only go down to ~$25 per node after the range is small enough for the
1 or 2 mw units.

Another factor:  I'm currently using CAT-5 because it's a "commodity" item
(LOTS of it is made and available) but I am currently only using one of the
4 pair in the cable.  If future use does not surface for the other 3 pairs,
I could go to a jacketed single twisted pair which would again lower the
cost for cable, or even, since it's in conduit, I could use single-pair,
un-jacketed cross-connect wire.

That said, I still see a use of wireless in my application, for example:
One of my users is located in a public park and they have a small number of
signal heads that, due to vandalism, must be put out when needed (let's say
two days per month) and gathered up at the end of each day.  I can see the
signal head containing a battery (recharged by just plugging the signal head
into a charging station at night), a node and Zigbee.  No need to run
data/power cable in the city park and no vandalism.  I can also envision
other cases where a network would be a mix of wired and wireless ...

Oh, one other advantage of Zigbee could be that (if self powered) it is
inherently protected from lightning, HOWEVER the node is still connected to
the rails which are still great lightning/ground voltage gradient attractor
:-)

Apologies to others not interested in such non-AVR discussions ... Now back
to our regularly scheduled AVR specific discussion ...
 
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