FA-92194 / PLC ladder logic scan cycles / Member archive

Alarm on-delay counts non-consecutive true scans · case 04

A chattering signal raises the alarm although it was never true for the required consecutive scans.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Digital alarm instruction evaluated per scan; scans is a list of [condition, ack, reset], min_scans >= 1. The alarm activates when the condition has been true for min_scans consecutive scans; activation clears the acknowledged flag. A non-latched alarm clears when the condition goes false; a latched alarm clears only on a reset while the condition is false. ack sets acknowledged. Return [in_alarm, acknowledged] per scan.

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: scenario 2	`[[false, true], [false, true], [true, true], [true, true], [false, true], [true, false], [false, false], [false, false], [true, false], [true, true], [true, true], [false, true], [true, false], [true, true], [true, true]]`	`[[false, true], [false, true], [true, true], [true, true], [false, true], [false, true], [false, true], [false, true], [false, true], [true, true], [true, true], [false, true], [false, true], [true, true], [true, true]]`	Failed

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