For STD-302Z and LMD-401, why should continuous Hi or Low input data be within 20 ms?

The receiver circuits use a circuit called a Slicer Circuit which consists of a comparator to take the analogue AF signal and determine the data pulses. The time constant in the slicer circuit limits continuous pulse widths to 20ms. If user data contains wide pulse widths, it is advised to implement dummy data or Manchester coding to limit the pulse widths.