FA-92307 / PLC ladder logic scan cycles / Member archive
Stack light flashes for an unacknowledged alarm of a hidden level · case 02
The red lamp flashes because of an unacknowledged low alarm that the lamp is not showing.
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 fixtureThis sample comes from the broken implementation of a controlled reproducer.
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| regression: scenario 8 | [["red", true, true], ["red", false, false], ["red", true, true], ["red", true, false], ["red", true, false], ["red", false, false], ["red", false, false], ["red", true, true], ["red", true, false]] | [["red", true, true], ["red", false, false], ["red", true, true], ["red", true, false], ["red", true, false], ["red", false, false], ["red", false, false], ["red", true, true], ["red", false, false]] | Failed |
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