X10 transmissions are synchronized to the zero crossing point of the AC power line. The design goal should be to transmit as close to the zero crossing point as possible within 200 microseconds of the zero crossing point.Typical X10 phase couplers provide a 60 Hz square wave with a maximum delay of 100 micro seconds from the zero crossing point of the AC power line. The maximum delay between signal envelope input and 120KHz output bursts is 50 microseconds.
A Binary 1 is represented by a 1 millisecond burst of 120 KHz at the zero crossing point and a Binary 0 by the absence of 120 KHz. An XM10 would modulate its input (from the OEM) with 120KHz, therefore only the 1 ms "envelope" need be applied to their inputs. These 1 millisecond bursts should actually be transmitted three times to coincide with the zero crossing points of all three phases in a three phase distribution system.
A complete code transmission encompasses eleven cycles of the power line. The first two cycles represent a Start Code. The next four cycles represent the House Code and the last five cycles represent either a Number Code (1 through to 16) or a Function Code (On, Off etc.). This complete block, (Start Code, House Code & Key Code) should always be transmitted in groups of 2 with 3 power line cycles between each group of 2 codes. Bright and dim commands are exceptions to this rule and should be transmitted continuously (at least twice) with NO gaps between codes.
Within each block of data, each four or five bit code should be transmitted in true and complement form on alternate half cycles of the power line. i.e. if a 1 millisecond burst of signal is transmitted on one half cycle, (binary 1) then no signal should be transmitted on the next half cycle, (binary 0).
The Start Code is always 1110, a unique code and is the only code which does not follow the true complement relationship on alternate half cycles.
1) Hail Request is transmitted to see if there are any other X10 transmitters within listening range. This allows the OEM to assign a different House code if a "Hail Acknowledge" is received.
2) In a Pre-Set Dim instruction, the D8 bit represents the Most Significant Bit of the level and H1, H2, H4 and H8 bits represent the 4 Least Significant bits
The Extended Data code is followed by 8 bit bytes which can represent Analog Data (after Analog to Digital conversion). There should be no gaps between the Extended Data code and the actual data, and no gaps between data bytes. The first 8 bit byte can be used to say how many bytes of data will follow. If gaps are left between data bytes, these codes could be received by X10 Modules causing erroneous operation.
3) Extended Code is similar to Extended Data: 8 Bit bytes which follow Extended Code (with no gaps) can represent additional codes. This allows the designer to expand beyond the 256 codes currently available.