Neil,
I just checked the F872 and it doesn't have a UART. ( I've never used a 
part that didn't have a UART since I begrudgingly deserted the old 5x 
stuff years ago!)  I'd bump up to the F873A and use the UART unless you 
have a closet full of F872s.  Monkeying around with any of the three 
ways you mentioned might get a bit tedious at those distances.  RS485 is 
easy, noise immune and cheaper.
Bill

PicDude wrote:

>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.
>
>
>
>
>
>  
>

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