The method used by X-10 is based on a simple data frame with eight data bits (one byte) preceded by a predetermined start code
The complicated part of this technology was not the system of binary data, but the method in which it was transmitted from one device (the transmitter) to another device (the receiver). The key was for every device to have an integral "zero crossing" detector so that all of them were synchronized together (figure 1). A receiver opens its receive "window" twice each sine wave (figure 2), that is 120 times each second or 7,200 times each minute. (ThatÂ’s 432,000 an hour, or 10,368,000 a day! That means itÂ’s looking for little pulses of data 3,784,320,000 times a year !!....at 60Hz, anyway)i
Since these devices would not have any direct wiring between them, it was necessary to devise a way of sending the data over the existing electrical wiring. The actual binary data is transmitted by sending 1ms bursts of 120kHz just past the zero crossing of the 60Hz power. (While North America remains the primary market for X-10 based devices, products are also available which are designed for use on 50Hz electrical distribution systems.) It was also obvious that complementary bit pairs were necessary. Therefore, a binary "1" was defined as the presence of a pulse, immediately followed by the absence of a pulse. A binary "0" was defined as the absence of a pulse, immediately followed by the presence of a pulse (figure 3)i
While the transmitted pulses were to be a full 1ms in duration, the receivers were designed to open a receive window of only .6ms. That allowed for the loose tolerances of the 1978-era components to "slop" plus/minus 200m sec.
In order to provide a predictable start point (figure 4), every data frame would always begin with at least 6 leading clear zero crossings, then a start code of "pulse", "pulse", "pulse", "absence of a pulse" (or 1110)i
Once the Start Code has been transmitted, the first nibble is sent. (If you are not familiar with the term "nibble", that means 4 bits or half a byte.) In order to make it easier for the consumers to operate the devices, this first 4-bits were given "letter" code designations (figure 5). It was also decided to randomly rearrange the patterns so that the "A", "B", "C" codes, etc., did not fall in the predicable binary pattern. It is easy to see that in reality, the "M" code is first in the binary progression
In one contiguous bit stream, the second nibble provides the second half of the address (figure 6). The last bit appears to be a part of the "number" code but in reality it is a function bit. Whenever this function bit is a "0", it designates the preceding nibble as a number code and therefore a part of the address.
For purposes of redundancy, reliability and to accommodate line repeaters, the X-10 protocol calls for every frame of data to be transmitted twice (figure 7)i
figure 7Whenever the data changes from one address to another address, from an address to a command, from one command to another command or from one command to another command (figure 8), the data frames must be separated by at least 6 clear zero crossings (or "000000"). When teaching classes in this stuff, I often say that this gap "gives the receivers a chance to catch their breath". In reality, of course, the sequence of six "zeroÂ’s" resets the shift registers
Once a receiver has processed its address data, it is ready to receive a command. As before, all data frames must begin with a start code. Then the following nibble gives the letter code (figure 9). The next nibble is the command. Since the last bit is the function bit (bf = 0 = address number, bf = 1 = command) all the commands end in a binary 1
figure 9This diagram (figure 10) only shows the six most often used commands. A later graphic will illustrate all the available commands. As before, all X-10 protocol transmitters send their data frames twice (figure 11)i
Figure 12 shows that an example transmission of two data frames (A1 A1 A-On A-On, for instance) would take 47 cycles of the 60Hz sine wave. That would equate to 0.7833 seconds, or in practical terms, just under 1 second. Of course, some commands take less time. When sending an "All-Lights-On" command, for example, no address needs to be sent. Therefore the entire two frame sequence takes only one third of a second (actually, 0.3666 seconds, but whoÂ’s quibbling). If your receivers react on the first frame, it could take a mere two tenths of a second (0.1833 seconds)i
Up to this time, all the diagrams have shown only one pulse but that is not entirely correct. I did that just to make it easier to explain. In reality, it is not a "single phase" world. On this planet, we generate our electrical power in 3 phases (figure 13) and so all X-10 compatible transmitters "should" send out 3 pulses (as in figure 14)i
Finally, I promised to give you an "introduction" into X-10Â’s Extended Code Protocol. IÂ’m going to take the easy way out and just give you first a graphic showing all of the available bit sequences in the X-10 standard code. Instead of making this just part of the text of this article, I have made it a graphic so the word-wrap feature of your browser wonÂ’t screw up the alignment
Almost everything I have said since the beginning of this explanation can be summed up in this one graphic, but arenÂ’t you glad I took the time to explain it?
You will notice that there are some changes in four of the command codes.
* Ext Code0111- Now designated as "Ext Code 1", for data and control
* Preset Dim (1)1010- Now designated as "Ext Code 3", for security messages
* Preset Dim (2)1011- Now designated as "Unused"
* Ext Data1100- Now designated as "Ext Code 2", for meter read and DSM
As far as we know (at the time of this writing) only "Extended Code 1" has a defined frame length which is 31 cycles (62 bits) and is described as:
* Start Code = 4 bits,
* Housecode = 4 bits,
* Extended code 1 = 5 bits (01111),
* Unit code (device code) = 4 bits,
* Data = 8 bits,
* Command = 8 bits..
The explanation for not having a defined frame length for the other two is:
"Extended code 2 is variable in length, depending on the type of message. It has its own separate "attention" marker to separate it from all other formats.
Extended code 3 has been "assigned" for security but doesn't actually exist yet so its format has not yet been defined."i
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