The Internet Power Line Adapter - University of Queensland

Home Automation, Power Lines and the Internet Quenten Alick
2.4.2 CEBus Transmission Protocol
The CEBus standard does not define exactly how each medium should represent
data. It defines that the transmission medium should be capable of taking on two
different states, superior and inferior. The inferior state is defined as the idle
state of the medium. To begin making a transmission, the medium starts in the
superior state. It then alternates states for each successive symbol that is
transmitted.
The length that each state is transmitted for indicates which symbol is being
transmitted. There are four different symbols, logical one, logical zero, end-of-
field and end-of-packet. The transmission lengths and symbols are summarized
in figure 2-4.
Symbol Name Transmission Length
Logical One 100μs
Logical Zero 200μs
End-of-Field 300μs
End-of-Packet 400μs
Figure 2-4 List of CEBus symbols & Transmission times
In the case of power line CEBus products, the superior state is defined as a
“chirp” of noise which is transmitted at the zero crossing point of the 50Hz AC
power signal. The “chirp” is defined as a signal that changes frequency from
200kHz to 400kHz, then 100kHz to 200kHz within the space of 100μs. The
spread-spectrum chirp is shown in figure 2-5. In order to create the different
length states for the different symbols, this 100μs chirp is repeated to create the
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