[sdiy] Sending Digital Commands Over Your House's AC Wiring?

[Scott Gravenhorst]

Ken Elhardt <ken.elhardt at gmail.com> wrote:
>I have a question that I've been wondering about for at least 25 years
>because it seems very bizarre and I've never heard of anything else on
>earth working this way.  I have an expensive Anova phone/control
>system from the early 80's.  In addition to the phone, it has has a
>security system unit that works with sensors placed about your home,
>and also a home appliance/lighting control unit for turning things on
>and off automatically.  To use that last unit, you plug these modules
>into the outlets, which they themselves have outlets on and you plug
>what you want to control into them.  You set the device number from 1
>to 16 for each of those modules.  The main control unit communicates
>with these modules that are plugged in all over your house by sending
>commands over the house's AC wiring.  I've never known AC to be used
>for anything other than power.  I just can't imagine how they're
>sending commands out of the AC power cord, into the house's wiring,
>that is then read by the modules.  Just as the manual says, if I try
>to plug the main controlling unit into an isobar with its filtering,
>it doesn't work, the modules won't respond to commands.  Technically,
>how are they doing this?  Imagine synths sending MIDI data to each
>other through their power cables.  That's how weird this is.
>
>Puzzled,
>-Ken Elhardt

I studied this when I had a similar system sold by Radio Shack - B-something. 
Anyway, an Oscope helped me learn what they did, it's serial data that is sent over
the mains synchronized to a zero crossing.  I was able to rewrite their Trash-80
Color Computer driver for a Radio Shack Model 100 system and could control damn
near everything in my house from a computer.  Ultimately, I gave up on that because
the modules that turned stuff on and off had a rather short lifespan of only
several months.  Worked great when it worked, but it sucked to find out - oh that
lamp won't turn on now...



-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.