Regarding Gen 1 base station modulation, both base stations modulate their lasers a about 1.8MHz continuous wave (CW). The X and Y rotors for each base station and the optical blink sync for each base station all modulate at this same frequency. So, the issue is how to disambiguate from what is the source of modulate light that a sensor just received. In the Gen 1 systems this is done with Time Division Multiplexing. Each base station takes turns with the other base station sweeping their lasers so the the lasers don't interfere with each other.
Sorry for the long intro,... so for Gen 1 base station, the output of the D pin on the TS4231 will be a one's and zero'es pattern corresponding to the CW modulation frequency of 1.8MHz. And, note the 1.8MHz isn't a forever fixed number. This carrier frequency could be anything between 1.8 to 5MHz. For current systems the modulate frequency is fixed at 1.8MHz.