The 74HC123 consists of dual retriggerable monostable multivibrators with output pulse width control by three methods:
- The basic pulse is programmed by selection of an external resistor (REXT) and capacitor (CEXT).
- Once triggered, the basic output pulse width may be extended by retriggering the gated active LOW-going edge input (nA) or the active HIGH-going edge input (nB). By repeating this process, the output pulse period (nQ = HIGH, nQ = LOW) can be made as long as desired. Alternatively an output delay can be terminated at any time by a LOW-going edge on input nRD, which also inhibits the triggering.
- An internal connection from nRD to the input gates makes it possible to trigger the circuit by a HIGH-going signal at input nRD as shown in the function table.
Schmitt-trigger action in the nA and nB inputs, makes the circuit highly tolerant to slower input rise and fall times.
The 74HC123 is identical to the 74HC423 but can be triggered via the reset input.