I'd go with the suggestion to use CAN - it's designed for just the sort of thing you are talking about, and 
it's very reliable and noise-tolerant.  It's not the cheapest way to go - you need a line-driver chip (eg. 
MCP2551) and either a PIC that has CAN already (eg. 18F258) or you need a CAN controller chip (eg. MCP2510 - 
note the close resemblence in part numbers - they do very different jobs!) at each point in the system.

The thing I noticed that rang a warning bell:  you are planning to use telephone cable, and include the power 
supply along it, I think?  You need to check the current-carrying capability you need, and the voltage drop 
you are going to get.  Unless the cable run is very short, and you said it wasn't, you will need to feed a 
high-ish voltage down the cable and regulate it at each unit.  Power Over Ethernet, for example, uses 48V fed 
through Cat5 cabling, but you can probably get away with 12V or so if your units need 5V.  (I hope you weren't 
expecting to put 5V in at the Master end and just pick it off at each point!  :-)

Cheers,


Howard Winter
St.Albans, England


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