X10 is a communication protocol for remote control of electrical devices. It uses power line wiring for signalling and control, where the signals involve short RF bursts representing digital information.

X10 was developed in 1975 by Pico Electronics of Glenrothes, Scotland, in order to allow remote control of home devices and appliances. It was the first domotic technology and remains the most widely available.

A number of higher bandwidth alternatives have been proposed, including INSTEON.

X.10 is an industry standard for communication among control devices for home automation.

Household electrical wiring is used to send an address and a command via a digital signal on 120 kHz, during the quiet zero crossings of the 50 or 60 Hz AC alternating current waveform. More advanced versions can also query devices to respond with their status, such as "off" or "on" – or even with the temperature or another sensor reading. Devices usually plug into the wall, where a lamp, television, or other household appliance plugs in; however some built-in controllers are also available for wall switches and ceiling fixtures.

The relatively high-frequency signal cannot pass through a transformer or across phases of power, so a repeater is sometimes used with a system. In order to block signals from getting in or out, an attenuator may also be used.

Protocol and system description

The X10 protocol consists of bit "address" and then bit "command". For example, you can say "lamp #3", "turn on!" - and it happens. You can address several units before giving a command: "lamp #3", "lamp#12", "turn on!".

Appliances are generally plugged into X10 modules (receivers). X10 makes a distinction between "lamp modules" and "appliance modules".

Appliance modules supply electrical devices with power and accept X-10 commands. Appliance modules are capable of handling larger loads (i.e. coffee maker machine, heaters, motors,...), simply turning them on and off.

Therefore, if you wished to control a light via X-10 commands, you would plug the light into a light module and the module into the mains. You would assign it an address (A1, for example). Now, when you sent the "A1 on" command down the power lines, the light would turn on. Lamp modules aren't capable of running heavy loads.

Each device receiver is set to a certain unit ID (X10 addresses), and reacts only to commands addressed to it. Receivers ignore commands not addressed to them. X10 allows up to 256 distinct devices (addresses) on a home power system: 16 unit codes (module numbers), 1- 16, for each of 16 house codes, A-P, (16 X 16 = 256). If 16 unit codes is not enough, even with sharing unit codes, you might consider using more than one house code.

The "base stations" (transmitters) was a simple control box that could be programmed to transmit various X-10 commands down the power lines. Modern methods consist of either using a RF remote control (that interfaces with a radio receiver) or via your computer using special software.

All commands are broadcast through the power mains therefore requiring no additional wiring. There are several commands, the basic ones being: on, off, all on, all off, bright, and dim.

In the 60 Hz AC power flow, a Binary Digit (bit) 1 is represented by a 1 millisecond burst of 120 kHz at the zero crossing point (0º, but certainly within 200 microseconds of the zero crossing point), immediately followed by the absence of a pulse. And a Binary 0 by the absence of 120 kHz at the zero crossing points (pulse), immediately followed by the presence of a pulse. All messages are sent twice to reduce false signaling. After allowing for retransmission, line control, etc, data rates are around 20 bit/s, making X10 data transmission so slow that the technology is confined to turning devices on and off or other very simple operations.

In order to provide a predictable start point, every data frame would always begin with a start code of "pulse", "pulse", "pulse", "absence of a pulse" (or 1110). Immediately after the start code, a letter code (A-P) is sent and after the code, comes a function code( number code,1-16, or command code, the selection of which is determinated by the last bit, 0=address number and 1=command). One start code, one letter code and one function code is known as a frame.

Each signal is also sent two times to make sure the receivers understand it over the "noise" of the power lines (for purposes of redundancy, reliability and to accommodate line repeaters).

Whenever the data changes from one address to another address, from an address to a command, or from one command to another command, the data frames must be separated by at least 6 clear zero crossings (or "000000"). The sequence of six "zero's" resets the shift registers.

X10 clocks

A X10 clock is a electronic clock that sends the actual data and time to the device timers. It generally includes some batteries to work when the power is off.

Weak points and limitations

One problem with X10 is excessive attenuation of signals between the two out-of-phase 120 volt lines used in typical North American residential construction. Often, there's simply no reliable path to allow the X10 signals to propagate from one phase wire to the other; this failure may come and go as large 240 volt devices such as stoves or dryers are turned on and off. When on, such devices provide a low-impedance bridge for the X10 signals between the two phase wires. This problem can be permanently overcome by installing a capacitor between the phase wires as a passive bridge for the X10 signals. More sophisticated installations install an active repeater device between the phases.

(In Europe, entire houses are typically wired from a single 240 volt mains phase wire so this problem does not occur.)

Other problems: TVs or wireless devices may cause spurious off or on signals. Noise filtering (as installed on computers as well as many modern appliances) may help keep external noise out of X10 signals, but noise filters not designed for X10 may also filter out X10 signals traveling on the branch circuit to which the appliance is connected.

Some X10 controllers may not work well or at all with low power devices (below 50 watts) or devices like fluorescent bulbs that do not present resistive loads. Use of an appliance module rather than a lamp module may resolve this problem.

X10 signals can only be transmitted one command at a time. If two X10 signals are transmitted at the same time, they will collide and the receivers will not be able to decode the signal commands.

Alternative technologies

• INSTEON -- (http://www.insteon.net) Dual-band technology (powerline + RF), backwardsly compatible with X10. Developed by Smarthome, Inc.
• ZigBee
• UPB -- (http://www.pcslighting.com/UPBMain.htm) A new powerline protocol that offers improved performance and reliability over X10.

External links

• http://www.x10.com/home2.html
• http://www.smarthome.com
• news:comp.home.automation
• CEBus and HomePNP
• http://www.geocities.com/ido_bartana/x10theor.htm
• http://www.hometoys.com/htinews/feb99/articles/kingery/kingery13.htm
• http://www.geocities.com/ido_bartana/rf_protocol.htm
• http://wish.sourceforge.net/
• ftp://ftp.x10.com/pub/manuals/technicalnote.pdf