Physical layer
The physical layer in an X-10 system transmits data over the power lines. The challenge is to send the signal reliably, with relatively low-power, inexpensive electronics. X-10 chose a simple scheme using a tone or its absence to convey a one or zero data bit, respectively, and uses redundant signaling to help improve reliability.
Figure 3 shows the fundamental approach. An X-10 transmitter will output a 120 kHz tone in a 1 ms burst starting when a rising line voltage crosses the zero point. The presence of the tone signals a one bit; the absence of a tone signals a zero. The burst is further transmitted two more times, at 1.047 radian intervals, to provide signals that will have appropriate zero crossing timing when bridged to the alternate phases in a three-phase circuit.
FIGURE 3: X-10 signal transmission
Figure 1-3 shows bursts at the rising voltage zero crossing points, along with bursts starting at the zero crossing for the negative-going power line voltage. The X-10 signal protocol sends the complement of the bit being transmitted at that time (and in the following bursts for the other line phases) to provide error checking at the receiver on the received signal. For example, if there is a tone at the rising voltage zero crossing, representing a one bit, there will not be any tone at the falling voltage zero crossing, and vice versa. This approach provides no error correction, but helps the receiver detect when a bit has been corrupted.
Timing the signal at the zero crossings simplifies the receivers, reducing cost, because the underlying 60 Hz carrier provides precise timing for the window in which the 120 kHz tone will arrive. The physical layer protocol requires that the tone burst start no more than 200 (microseconds) after the zero crossing, defining the limits within which a tone detector must operate.
Because the system only transmits one bit per cycle of the underlying carrier, the raw signaling bit rate of the X-10 system is 60 bits per second (bps). The actual payload data rate is less, as shown in the next section. Because of the very low data rate, and because the physical layer has no provision to detect or avoid collisions, it’s important that X-10 messages be short and infrequent.