FA-92305 / PLC ladder logic scan cycles / Member archive

Horn silence is not cancelled by a new alarm · case 05

Once silenced, later alarms never sound the horn.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Stack light and horn logic. alarms is a list of [name, high|low]; each scan gives the active alarm names plus optional ack and silence buttons. An alarm becomes unacknowledged on its inactive-to-active transition and stops being unacknowledged when it clears or ack is pressed. Any new activation cancels silence; silence (pressed after that) mutes the horn. The lamp is red if any high alarm is active, else amber if any alarm is active, else off; it flashes if an unacknowledged alarm of the displayed level exists. The horn sounds while any unacknowledged alarm exists and not silenced. Return [color, flashing, horn] 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 5	`[["red", true, true], ["red", true, true], ["red", true, true], ["red", true, true], ["red", false, false], ["off", false, false], ["red", true, false], ["red", true, false], ["amber", false, false], ["off", false, false], ["off", false, false], ["off", false, false]]`	`[["red", true, true], ["red", true, true], ["red", true, true], ["red", true, true], ["red", false, false], ["off", false, false], ["red", true, true], ["red", true, false], ["amber", false, false], ["off", false, false], ["off", false, false], ["off", false, false]]`	Failed

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