A single TW523 powerline interface can be shared among several controllers, by using a splitter. These splitters look like regular telephone line splitters and are (Dec 2003 - this was available in two-way, three-way and four-way versions but its been discontinued). They are made by Smartlinc, and regularly cost between $11 and $16. The description says that "Solid-state electronics neatly hidden inside the splitter allow you to share a single TW523 with up to four devices with no loss of functionality."

Well, the truth is you can make your own splitter for a fraction of the cost. The "Solid-state electronics" amount to no more than a few diodes.

A tiny bit of theory: a regular telephone splitter simply splits each of the four wires of the plug to the four (or three or two) jacks, as depicted in the picture below. Each of the jacks gets one wire from each color.

Now, in the TW523, the wire number 4, the black wire, is the wire which the hosts sends data to the TW523. What we want to avoid is that this data will collide and enter the wire coming out of a different host. Thus, we use diodes to isolate the hosts from each other, like so:

This will prevent a signal path from a jack to any of its neighbors.

Implementation:

1. Get a regular telephone line splitter and T4148 diodes. You'll need a diode for each way in the splitter (two diodes for a two-way, etc.)

2. Carefully open the two halves of the splitter shell. You'll find a small PCB inside.

3. Look at the #4 wire. You'll need to cut the traces that connect the plug end to the jacks end. See the picture below, which shows a four way PCB.

4. Next, for each way, solder a diodes between the jack end and the plug end. The band of the diode is towards the plug side.

5. Finally, assemble the shell back, and that's it.