Hi all,

Looking for a simple way to have multiple PICs communicate with each other 
over distances of up to a couple hundred feet (which conservatively includes 
the vertical lengths up to the ceiling).  Currently I'd say about 10 rooms, 
but expandability to say 16 rooms would be nice.  Data speed is low (each 
message will only be a few bytes long, and messages should not exceed 1 per 
minute).  But to reduce wiring, I'd like to use a bus type system, rather 
than a star-type configuration with a central room acting as a hub.  With the 
bus type system, I will designate one PIC as the master and the rest as 
slaves.

>From what I understand of SPI, I will need a separate slave-select line for 
each slave, so that adds more wires.  My understanding of I2C is that it is 
truly a bus system -- just 2 wires can be shared among all PICs.  Add power 
and ground, and I can use plain jane 4-conductor telephone wire for this -- 
nice! Another benefit is that I would like to use PIC16F872's all around, and 
these have I2C built in.

The issue though, is that I2C has a limited distance, though I understand that 
at lower speeds, I can get more distance.  But how much?  I have yet to learn 
the specifics of I2C, including the various speeds available, but at a high 
level, would a couple hundred feet reliably be possible for my snail-like 
data-rate?

One option to solve this, is to add on one of Philip's I2C bus extenders such 
as the P82B715 ($3.38 ea from Digikey, and available in picdude-friendly SOIC 
packaging).  Would this extender be necessary for my app?

I've also looked at 1-wire, which is appealing, but implementation looks more 
time-consuming, since the PIC does not have 1-wire support built in.  Sure, I 
can code it in, but simplicity is an appealing factor for this one-off 
project.

Cheers,
-Neil.





-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist