One of my customers reported a problem using the XTB-523 as a powerline interface for the RCS X10-compatible thermostat. I acquired one of the RCS thermostats to investigate the problem, and discovered it was not adhering to the X10 protocol spec.
The X10 spec says the maximum allowable delay from the 60Hz zero crossing to the assertion of the X10 envelope input from the OEM product (RCS thermostat) is 50uS. The XTB-523 sampled that input for about 250uS before switching to monitoring the powerline. The RCS thermostat that I tested asserts its output 200uS after the falling edge of the zero crossing, and 250uS after the rising edge of the zero crossing. Since that is at the borderline of the XTB-523 sampling that input, some of those pulses were missed.
To accommodate the delayed output from the RCS thermostat, I modified the code to continue monitoring the digital port during the X10 reception window. Please contact me directly if you are having trouble using the XTB-523 with either the RCS TX10-B or RCS TX15-B thermostats.