As with many homes, there are two pair of telephone lines that run though a common cable and connect to telephone jacks throughout the house I live in. The problem with ADSL, at least with the True and TOT ADSL lines I've had in Thailand, is that if a telephone is connected directly across line, the ADSL data connection becomes unreliable, so it is necessary to use the splitter that came with the modems. When I had a TOT line with ADSL installed in a house with telephone jacks throughout the house, the installer said that I would have to use an ADSL splitter on each telephone. Ok, I will. But rather than using a splitter for each telephone, I took advantage of the fact that the house was wired for two telephone lines and have one splitter shared among all the telephone jacks in the house.

The telephone jacks are wired for two lines, designated line one and line two. Single line telephones are designed to connect to line one, as defined pins on their RJ-11 connector they use. The ADSL Splitter Sharing Adapter takes the "raw" telephone line in on line two, and puts the split off telephone signal onto line one.  The modem plugs into the splitter as normal, which means that the ADSL Splitter Sharing Adapter must be used on the phone jack in the room that the modem is located in.

When  the telephone line was first installed, the installers connected the red and green wires to the line. The red-green pair of wires correspond to line 1. Later, after I had built the ADSL Splitter Sharing Adapter, I disconnected the red-green pair of wires and connected the yellow-black pair of wires in their place. Inside my office, the yellow-black pair of wires connects to the input of the ADSL splitter, and the "telephone" output of the splitter connects to the red-green pair of wires.  If two telephone lines were to have been installed, a two line terminal block would have been used, and one telephone line would have been connected to the red-green pair or wires and the other telephone line would have been connected to the yellow- black pair or wires.

The color of wires used in RJ-11 telephone cables vary, but some of the cables I have use the colors in the illustration above. The colors correspond to the colors of the wires that run though the walls. By convention, the outside pair of conductors, the yellow-black pair, is referred to as line two, and the inside pair of conductors, the red-green pair, is referred to as line one. By convention, the connector on a single line telephone is wired to connect to line 1.

The physical basis of the adapter is a four wire telephone jack bought at a hardware store, and two two-line telephone cords with RJ-11 connectors. The two cords were obtained by cutting a telephone cord with RJ-11 connectors on each end in two. A two line telephone cord can be indentifed by the presence of four wires visible in the RJ-11 plug. Some cords only have two wires, which are connected to the two inside pins on the RJ-11 connector. Although it the electrical connections could have been made using the screw terminals provided, I ended up soldering the connections because the plastic was so soft, I couldn't put much torque on the screws without stripping the threads. Plastic cable ties were used for strain relief.

The wiring is pretty straight-forward. Though colors in the telephone cords may vary depending upon the manufacturer, the inner two conductors on the connector seem to always be line one and the outer pair seem to be line two.  In the wiring diagram above, yellow and brown are line two, and red and green are line one. You may find that for some reason, you will have to swap the red and green or yellow and black conductors in some installations.