Introduction / Context:
The 74123 is a retriggerable monostable multivibrator (one-shot). Its defining feature is that an additional trigger received while the output pulse is active will extend the timing interval. This behavior contrasts with a nonretriggerable device, which ignores triggers until the interval ends.
Given Data / Assumptions:
- Device type: 74123 retriggerable one-shot with R–C timing.
- New triggers can occur any time, including during the active pulse.
- Pulse width nominally T = k * R * C, where k depends on device specifics.
Concept / Approach:
In a retriggerable one-shot, a valid trigger resets the internal timing capacitor near the start of the cycle, effectively adding time from the trigger instant. If triggers are spaced closely, the output can remain asserted for a long period, because each new trigger extends the pulse. Therefore, the claim that it “must be started all over again” after finishing is misleading: it need not finish before being re-triggered; it can be extended mid-cycle.
Step-by-Step Solution:
Apply a trigger at t = 0; output goes HIGH for duration T.Apply another trigger at t = T/2; the device extends the pulse roughly to t = T + T/2 (depending on internal reset timing).Continue periodic triggers; output stays HIGH so long as retriggers arrive before the end of the current interval.If no trigger occurs and t reaches T, output returns LOW and the device awaits the next trigger.
Verification / Alternative check:
Timing diagrams in references show staircase-like extensions of the HIGH interval with each retrigger.
Why Other Options Are Wrong:
Correct: Implies the device cannot be retriggered mid-pulse, contradicting its core feature.Only true for negative-edge triggers: Polarity selection does not change retriggerable behavior.True if R–C values are small: R–C values set T but do not change the retriggerable nature.
Common Pitfalls:
Confusing 74121 (nonretriggerable) with 74123 (retriggerable).Forgetting to meet input qualification and pulse-width requirements for valid triggers.
Final Answer:
Incorrect
Discussion & Comments