Acutally, the IR543 (aka 4040) is an IR to X-10 power line signal transceiver. It doesn't respond to, or transmit RF singals. I receives X-10 IR signals, and converts them to X-10 power line signals on your home's AC power line.
The IR543 transceiver is necessary to use a Pronto to control a powerline X-10 receiver (e.g., wall switch, plug-in modules, etc.) We stock the IR543's and normally ship them out the same day ordered.
You can also use the Palm Pad and RF transceiver to control X-10 receivers via RF.
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