Here is a "bullet proof" manufacturer of
line carrier products.
http://www.digitalavnew.com/UPB-FAQ.htm
An irrigation specialist told me about this company. They use this product to control valves and pumps on large projects where someone forgot to run control wires. I have used this product before. It is expensive (at least 4 times X-10) and takes some time to learn how to program and set-up but it actually does the job every time. Check out the following from their website.
Q: What are the technical differences between UPB and X10?
A: Although UPB and X10 each use standard powerlines to communicate, the underlying technologies of each are significantly different. First, X10 uses a fixed carrier frequency to communicate data. Carrier frequencies traveling on household power wiring are subject to attenuation and noise interference, which can cause intermittent reliability issues. UPB utilizes a patented communications method called Pulse Position Modulation (PPM) that is based on timed pulses rather than a carrier frequency. Using pulses for signaling means (a) less attenuation on the powerline and (b) less susceptibility to RF interference. In addition, there is a difference in voltage strength: an X10 signal is 4 volts at its strongest level; while UPB is 40 volts, or 10 times stronger. The end result is faster and more reliable communication, especially when transmitting over longer distances. Another difference between the two technologies is that the transmission packet for UPB allows more data to be communicated over a much wider address space. This enables a higher level of control and far less potential of interference between adjacent homes. Lastly, UPB is a two-way communication protocol, whereas X10 is primarily unidirectional. Two-way communication allows the system to verify that a given message was successfully received.
