We did something similar a few years ago, except that instead of running two twisted pairs we used 18 gauge thermostat wire for the power, and ran 24 volts alternating current, supplied by a standard HVAC transformer. Each node then had it's own 7805. The SN75176 requires about 30 milliamperes for itself, plus the PIC and any additional circuitry. We kept the power wire separated from the data wire as much as possible. Some of our runs exceeded thirty meters between nodes, and voltage drop was noticible when using a smaller gauge wire.

Parallax has a nice little application note available at;
http://www.parallax.com/dl/docs/prod...munication.pdf
They also have an example program written for the Basic Stamp that should get you started. If you keep your data rate low you may get away without terminating resistors, but anything above around 9600 baud and you will likely need them.

Jerry.