The 74HC423 is a dual retriggerable monostable multivibrator with output pulse width control by two methods. The basic pulse time is programmed by selection of an external resistor (R_EXT) and capacitor (C_EXT). Once triggered, the basic output pulse width may be extended by retriggering (nA) or (nB). By repeating this process, the output pulse period (nQ = HIGH, nQ = LOW) can be made as long as desired. When nRD is LOW, it forces the nQ output LOW, the nQ output HIGH and also inhibits the triggering. Schmitt-trigger action in the nA and nB inputs, makes the circuit highly tolerant to slower input rise and fall times. The '423' is identical to the '123' but cannot be triggered via the reset input. Inputs include clamp diodes. This enables the use of current limiting resistors to interface inputs to voltages in excess of V_CC.