XIN/XOUT receives or sends commands from/to the X10 module.
From the manual:
XOUT is used to send control information to X-10 modules. These modules are available from a wide variety of sources under several trade names. An interface is required to connect the microcontroller to the AC power line. Either the PL-513 for send only, or the TW-523 for two-way X-10 communications are required. These devices contain the power line interface and isolate the microcontroller from the AC line. Since the X-10 format is patented, these interfaces also cover the license fees.
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