The 74LS123 is a dual retriggerable one-shot or monostable multivibrator with a 100% duty cycle and the ability to produce output pulses with durations ranging from a few nanoseconds to extremely long. There are three inputs on each device, allowing users to select either leading edge or following edge triggering;

• Pin (A) is an active-low transition trigger input
• Pin (B) is an active-high transition trigger input.
• Clear (CLR) input terminates the output pulse at a predetermined time independent of the timing components.

The clear input also serves as a trigger input when it is pulsed with a low-level pulse transition.