An astable multivibrator can be synchronized to an external chain of pulses. A bistable multivibrator implemented as a comparator with hysteresis. This repeats and forms a free-running oscillator or an astable multivibrator. The circuit can be thought as a 1/2 astable multivibrator. He and Bloch also invented the astable multivibrator. Alone, the trigger is a bistable multivibrator. A two transistor multivibrator module drove the audio alarm thru an amplifier. If further trigger pulses do not affect the period, the circuit is a "non-retriggerable" multivibrator. They called their circuit a "multivibrator" because its output waveform was rich in harmonics. This is a non-electronic type of multivibrator. In this mode 8253 can be used as a Monostable multivibrator.