Serial ports can only receive serial data that is correctly formed into a series, with an appropriate start bit, a series of data bits (usually 8 of them), and a the stop bit. Even within that format, decoding of the pulses is determined by voltage levels, with voltages below -3V being a bit representing logic 1, and voltages above +3V being a bit representing logic 0. By agreement, least significant bit is sent first.
Thus, if you have a train of pulses none of which -3V or lower, then no start bit will ever be deemed to have been seen, and no data will be deemed to have been transmitted. If no pulse is above +3V then no stop bit will be deemed to have been seen. The required timing of the pulses depends upon the agreed serial port rate.
