The digital signal is sent one bit at a time at zero crossing on the power line. What zero crossing means is that the transmitter waits for the 117 VAC, 60 Hz sine wave to be close to or at zero volts on the sine wave to transmit a bit of its digital signal. At the zero crossing moment the power line is its quietest and the digital bit will have the least noise to contend with to be received without error(s).
The bit is sent as a low voltage 120 KHz one-millisecond pulse on the power line. Each pulse (or absence of pulse, bit = 0) sent at the zero crossing moment is a bit. The receiver always listens on the power line for X-10 commands. The beginning code of the transmitted signal is the address of the receiver for which the command is intended. When the receiver hears its address being broadcasted on the power line, it responds to the following X-10 command.
Addresses: House Codes and Key Codes:
The house code and key code make up the “unique” address for each appliance module. House codes are the 16 letters A through P, and Key codes are the numbers 1 through 16. The appliance module has two rotary switches to set both the House Code and Key code for that module, see figure 2. Doing the math for the different addresses we can generate, we can calculate 16 x 16 = 256 unique address combinations. So one could put 256 different appliance controllers on the power line and access them individually.
Once the address is set, the appliance module will only respond to X-10
commands sent to that address and ignore all others.
Connection to the Power Line:
Connecting to the power line of the home to transmit X-10 commands has
been made easy with the use of the PL513 module. The PL513 plugs into
a standard wall socket. It uses a standard modular phone jack (RJ11) to
connect to our circuit. The four lines in the RJ11 connector are; a zero
crossing detector output, zero crossing common ground, X-10 transmit line
and X-10 transmit line ground, see figure 3. When the transmit line is
brought high, the module outputs a 120 KHz signal on the power line. Our
microcontroller pulse this line to send out binary numbers.