In SX Microcontrollers, SX/B Compiler and SX-Key Tool, johncouture wrote:

RS485 is very kool stuff.  

To answer your question, I'm pretty sure that the way the RS485 protocol works is that it just checks the voltage difference between the two wires and each end has its own power supply and its own ground reference.  This is why RS485 works for such great distances (at least that is the way I understand it ... not that it is right :-)

The RS485 chips (SN75176) are cheap (i.e. Jameco $0.23 USD each) and you need one on each end.  You can run a signal over twisted pair for a couple thousand feet in the kHz range.  I have a few that I play around with and they are well explained in Jan Axelson's book "Serial Port Complete" (ISBN 0965081923 )

Quadracs (about $1.20 USD) are a little more difficult to find information on.  You see references to Triacs and SCR's but Quadracs aren't used in a lot of hobby projects that I've been able to find.  They are a three pin TO-220 devices that can be, in theory, used as a variable motor controller.  I've yet to get them to work with my surplus TLP630 (Darlington Optocoupler)  to get a true ON/OFF for 120V fan motor with optical isolation.  My ultimate goal is to try and build an inexpensive speed control for a ceiling fan that doesn't make noise (cheap ceiling fans, the kind I buy, are generally noisy at lower speeds .... tough to sleep under).

Anyway, here are some reference materials if you are interested.

Datasheets
   SN75176 (RS485 Transciever)
   http://focus.ti.com/lit/ds/symlink/sn75176b.pdf

   TLP 630 (DIP8 Darlington Optocoupler)
   http://rocky.digikey.com/WebLib/Toshiba/Web%20Data/TLP630.pdf

   Quadrac (A souped up Triac)
   http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/160240.pdf

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