Hardware Reference
In-Depth Information
appliance modules will also ignore the “all lights on” message but honor the “all units off,” which is suggested by the
subtle wording of the commands differentiating between lights and units . It is interesting to note that their inverse
variants (“all lights off ” and “all units on”) do not exist. This is intentional. One of the intentions of “all lights on” was
to act as a security feature. An accidental invocation of an “all units on” command might start a teakettle dry boiling or
something similarly dangerous. Conversely, “all units off ” provides a quick closedown procedure for the house.
Once the message has been sent, nothing else happens. Ever! The receiver does not generate an acknowledgment
of the message, and the sender doesn't query the state of the recently controlled device to confirm its arrival. This is
because the transmitting circuits are more complex and expensive than the receiver and because adding a message
facility would add cost and bulk to the simplest of light switches. Some two-way switches do exist, providing a way for
you to query their state, but they are more expensive.
However, in an attempt to ensure data validity, the message is sent twice, and both messages are compared for
equality since electrical noise on the power line could have corrupted part of the signal. Consequently, it takes around
0.64 seconds for an X10 message to be received. Although this is an accepted facet of the protocol, it is not particularly
friendly when guests are staying at your house, because when they try to turn on the light, it appears to have not
worked . . . so they press the switch again and in doing so turn it off! To overcome this, many devices have a local
switch that affects the light directly without sending an X10 message to do so. This is mostly true for X10 light switches
that act like a normal in-wall switch but not an in-place X10 socket that is controlled by an existing (that is, normal)
light switch.
Another problem that can occur with X10 is that of dead spots , where all messages can (and sometimes do) get
swallowed because of the electrical noise generated by certain appliances. The power supplies for some MacBooks
are known to have this issue. It is therefore sometimes necessary to move X10 devices to different sockets for them to
work. X10 signals are also lost when there is a transformer in the circuit or you have a split phase system. Again, you
may need to move both the transmitter and the receiver to the same side of the problem device.
N Before committing to an X10 installation, experiment with a couple of devices to ensure there is a location in
the house that is capable of issuing an X10 message that can get heard in the vital majority of other areas.
Note
General Design
Before buying and installing any devices, you must first consider what devices you want to control and how you
want to control them. The important part of that question is not how many devices you will use but how they will be
controlled. This can be as simple or as complex as you like. And there need not be a computer involved at all.
Simple Case
In this situation, your appliances will be controlled either by their local switches or by one or more wired controllers
plugged into the mains. A wired controller is necessary here because you always need some way of introducing the
X10 signals to the power line. There are some wired controllers (SD7233), which include timing circuits so they can
automatically turn the lights on or off at particular times of day—sometimes within a randomized time frame to confuse
potential burglars. These work well and provide a cheaper alternative to running a computer all day, every day.
Other than the basic timer functions, this setup can only be controlled by a human making physical contact with
the controllers. It is the cheapest way to begin an exploration into X10, but appliances cannot be controlled remotely
via web sites or e-mail or wirelessly from handheld controllers.
If aesthetics are important, there are some controllers (for example, TMD4, shown in Figure 1-11 ) that will fit
into a wall outlet, allowing you to use the existing light switches to control multiple lights without a Star Trek -like
controller on the coffee table. However, this requires the purchase of both an X10 switch (to send the message) and an
X10 light fitting (to respond to it) and is usually overkill for such simple setups.
 
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