Matt. The IR543 will control up to 16 unit codes (e.g. lights) on one house code. If you want to control more addresses than that, it would mean that you need to use another house code. There are 16 house codes, and 16 unit codes on each house code. This gets tricky using IR with X-10. The IR command set is not specific to any house code, but it is specific to unit codes. What determines the house codes is what you set the wheel on the IR543 to (A through P). So, if you point your Pronto at 2 IR543s set to different house codes and press 1 ON, both IR543s will simultaneously send X-10 signals on different house codes that will collide and defeat each other. If you use X-10 RF signals this isn't a problem because both the transmitters and receivers are set to specific house and unit codes so they maintain separate communication channels.
There are some "work arounds" if you want to use 2 IR543s in the same room. One is to place them in different locations so that when you transmit an IR signal from your Pronto, you can control which IR543 receives the signal. Other "work arounds" involve using a home automation controller to intercept the IR signal and trigger a macro. With a home automation controller there isn't a limit to how many lights you can control. The Pronto can transmit individual X-10 unit codes without corresponding ON/OFF or DIM commands. For example, Pronto could send "123" which could turn on a specific light. It could then send "321" which could turn off that same light. How this works is that the home automation controller sees "123" and that triggers a macro to turn on that lamp, perhaps to 30% dim.
RCI Automation[Link: ourworld.compuserve.com]