FA-92063 / PLC ladder logic scan cycles / Member archive

Overlapped periodic triggers are queued behind the running execution · case 03

Start times drift off the period grid after an overrun because missed triggers run late.

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Case contract

Periodic task triggered every period ms from t=0; exec_times[k] is how long the k-th triggered execution would take. A trigger arriving while the previous execution is still running (trigger time < completion time) is an overlap: it is counted and skipped, not queued; a trigger exactly at completion starts normally. An execution longer than watchdog raises a major fault at start + watchdog and scheduling stops. Return [start_times, overlap_count, fault_time or None].

Why this case matters

Ladder programs are executed as repeated scans; each defect here changes what a rung, timer, counter or data-table instruction reports on a particular scan, which is how commissioning and field faults are actually observed.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: skip then drift	`[[0, 25, 26, 30, 40, 50], 2, null]`	`[[0, 30, 40, 50], 2, null]`	Failed

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