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

[Ken Elhardt]

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