The TW523 is about as dumb as dumb can get. Not only does it not "only transmit when no other devices are talking on the powerline", it delays it's output so that you are actually seeing what was on the powerline 22 half-cycles earlier. Think of it as a 22-bit shift register with a 120Hz clock.
There is no way to tell whether the powerline is clear before transmitting. The best you can do is to check that the output is the same as your input but this is after the fact.
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