Some people report random difficulties with their X10 systems.  If signal strength issues have already been addressed through couplers and filters, then the remaining problems are often a result of collisions between X10 signals sent over the powerline.  This tutorial will explore this problem, and offer a few suggestions on how to minimize it.

Background

X10 data is sent over the powerline as a series of 1 mS bursts of 120KHz just after each zero crossing of the 60Hz waveform.  Some X10 transmitters output three bursts, aligned with the zero crossings of a 3-phase power distribution system.  Presence or absence of a burst signifies a logic "1" or a logic "0" respectively.  A standard transmission is comprised of 44 bit positions spread over 22 cycles of 60Hz.  This is organized as a 4-bit "1110" start pattern followed by 9 data bits transmitted in complimentary pairs.  This pattern is then identically repeated for the second half of the message.  More detailed information on the protocol is available directly from the X10 website in a .pdf document titled X10 Communications Protocol.  (It is 1.3MB, and will take time to download if you have a slow connection.)

When two X10 signals are transmitted nearly at the same time, some portions of those commands can overlap, causing a "collision".  Since X10 databits are transmitted as complementary pairs, half of the "bit positions" do not contain a 120KHz signal burst.  A collision is detected when some of these blank bit positions contain signal bursts from another transmitter.  That causes an error in the count of complementary bits, and the data should be rejected.  When just a single controller is used in a X10 installation, collisions can only happen in special circumstances.  There are several X10 modules that have two-way capability, and will transmit when their status changes.  If the transmitter does not incorporate collision avoidance, then it is possible for a random status change to conflict with another X10 command.

 

X10 3-Burst Complimentary Pair (with 120V AC waveform removed)

RF Transceivers

Repeaters

Noise Generators

How to Minimize Problems

I hope sharing my experience in these tutorials will help others obtain the same level of reliability that we have here.  X10 has been with us for 3 decades.  Its low cost and rich selection of devices still makes it a cost-effective solution.  Installations today can certainly be more challenging than they were decades ago, but investing some time and effort up front will give a big payoff in the years to come.