Let's start with a basic introduction to the X10 protocol. This KnowledgeBase will not go into the technical details about how X10 signals are transmitted onto the house wiring and the timing sequence involved when transmitting information. If you are interested in more details about the protocol, please go to the following web page: X10 Transmission Theory
X10 Powerline Communication In A Nutshell
In the past, if you wanted to be able to control certain lights and appliances from one location, you would have to run hundreds of feet of wire to route the power switches of each device to one location. Using X10 modules, you don't have to do this, since the X10 House/Unit codes and commands are transmitted over your existing house wiring.
House And Unit Codes
In order to control specific devices, all modules are assigned an address, which consists of a House and Unit code. There are 16 House codes (A through P) and 16 Unit codes (1 through 16). Each House code has 16 Unit codes. This means there are 256 possible addresses. House/Unit codes are referred to in the following manner:
A5, C7, M13, P4, etc.
When you wish to turn on an X10-controlled lamp, you have to tell the Lamp Module controlling that lamp to turn on. The Lamp Module is monitoring the powerline for a command specifically addressed to it. In other words, any command sent must be preceded by an address matching the Lamp Module's address. Let's say the Lamp Module's address is set to A5. The Lamp Module will not respond until it "sees" the A5 ON command on the powerline.
Note: There are exceptions to this rule. Most modules will respond to special commands that are addressed to a large group of modules. One example is the All Lights On command. Any lamp modules set to a specific House code (i.e. House code "A") will turn on when this command is transmitted.
We understand that everyone has a different way of learning, so these concepts may be difficult for some people to understand. If so, let's try looking at this a different way.
Imagine you have a room that contains groups of people. In fact, there are 16 groups of people. In each group, there are 16 people. You are at the front of the room and your job is to issue instructions to these people. Some of the instructions you are going to issue are for specific people and others are for entire groups of people. To prevent miscommunication, every group has been assigned a letter (A through P), and each person has been assigned a number (1 through 16). Each person wears a sign around their neck that has the letter and number they have been assigned.
Your first instruction is for a man you had spoken with earlier that day. His sign reads "A7". To ensure that he is the only one to respond to your instruction, you say, "A7 take one step forward." The man wearing the A7 sign obeys your instruction. No one else in the room follows this instruction. This example parallels commanding a specific X10-controlled lamp or appliance module (set to A7) in your home to turn on or off.
Your second instruction is a little different. It involves having all the women in the "J" group sit. To cause this to happen you say, "All women in the 'J' group sit down." Every woman in the "J" group sits on the floor. This example is comparable to the All Lights On. Lamp Modules assigned to a certain house code (in this case J) would respond. Lamp Modules assigned to a different house code would not respond. Appliance Modules do not respond because they are a different type of module.