Appliance Control: Making Things Do Stuff - Smart Home Automation with Linux and Raspberry Pi - page 2

X10 works by encoding the data in high-frequency bursts (of 120KHz) and adding it to the existing power
line. Because the mains supply in all countries is either 50Hz or 60Hz (with Japan and Tahiti using both!), these
high-frequency signals are customarily lost by most devices that are looking only to consume power. On the other
hand, a special device can be plugged into the power line that is interested in high-frequency bursts. It is consequently
possible to recognize one binary digit of data every time the voltage goes from positive to negative, or vice versa.

N Several devices are available that are based on this principle, with most do-it-yourself (DIY) stores stocking
their own variant. If they do not contain the X10 logo, however, they are not compatible with X10 because their protocols
differ. They can also conflict with each other.

Caution

Every device that is to be controlled by X10 must have an address. This address comprises two parts: a house
code and a unit code. The house code is simply a letter, from A to P, and should be unique to your house. Obviously,
with only 16 letters to choose from, the house code won't be unique to every house in the world, but it should be
unique to any property that shares your immediate mains supply. This usually comprises your neighbors, and
occasionally the property two or three doors down, because all your power lines converge in larger conduits under
the road. Consequently, any house that shares these lines will also share X10 messages, making it possible to control
your neighbors' appliances as well as (or instead of ) your own. Currently, few enough people are involved in home
automation (and specifically X10) for this to be a practical issue. You can provide yourself with some peace of mind right
now by placing a filter between the electricity meter and the rest of the house mains. This is usually called a whole house
filter , and several makes and models exist, such as the PZZ01, which permits 200A of current. Naturally, with the levels of
current involved (and the law in certain countries), many people hire a qualified electrician to install such a device.
The second part of the address is the unit code , of which there are 16, and is represented by a hexadecimal digit
between 0 and F. Although this might not seem a lot, 16 devices allows you to have two appliances (one light and one
other) in every room of a moderately sized four-bedroom house. Most rooms will have only one—the light—while
appliances such as TVs and radios are more likely to be effectively controlled through infrared or even Ethernet.
In addition to an address, every X10 receiver module fits into one of two broad types, either lamp or appliance .
This is a difference that exists in the X10 module itself and that governs how it will deliver power to the device plugged
into it and which messages it will accept. An appliance module simply provides on/off control to whatever is plugged
into it and usually has a high enough power rating to accept most household appliances (ovens excepted). In contrast,
a lamp module will also respond to brightness control messages, varying the voltage applied to the light bulb plugged
into it. Consequently, plugging a toaster into a lamp module can be problematic and a potential fire risk. Adding a
light to an appliance module, on the other hand, works fine and only suffers the limitation of losing the dimming
functionality.

N Some types of light (such as fluorescent and power-saving bulbs) cannot generally work on lamp modules and
must be used with appliance modules.

Note

Each X10 message consists of three parts:

u A start message block (a nibble of 1110)

u An address (a house code and/or unit code)

u A command code (for example, “switch on”)

There are several different commands, fitting mainly into two groups—house code messages directed toward all
devices and unit code messages targeting a single appliance. As mentioned earlier, each X10 module is built to accept
or ignore specific messages, usually according to whether it's designated a lamp or appliance module; however,