No need for a roll-your-own.  Why reinvent the wheel?  The obvious choice
here is probably RS485 multidrop.  Look at the spec's for the LTC485 line
driver chip.  You can drive this from 3 pins in half duplex mode.  It is
rated at 50ft max length for 2.5M/Bit operation, but because you are using a
much lower baud rate, 60ft should be no problem whatsoever.  It will work
over twisted pair (preferred), minimum spec is Cat3.

I may be corrected by the more knowledgeable on the list, but this would be
my choice.

Rgs
Ian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Phil
Sent: Tuesday, 25 February 2003 2:44 pm
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC}: Low speed/long distance communications


What would you suggest for serial communications between PIC's
(16F877/876's) that are located about 200 feet (60 meters) apart and on
Cat 3 (or if I am lucky Cat 5) cable?  The data rate is slow, much less
than 1,000 bps.  I have looked at I2C but it appears to be limited to
only a few meters.  Did I miss something in the spec that allows it to
operate at longer distances and reduced data rates?

I would prefer a "daisy-chain" interconnect but if necessary a star
topology could be implemented.

Is it time for a roll-your-own interface?  Currently I am not I/O
limited but that may change :~)

Data, Clk and Vss would seem to be the easiest and minimum number of
signals.

  Your thoughts and suggestions are appreciated.

Phil

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