FA-91874 / PLC ladder logic scan cycles / Member archive

TOF output energizes at power-up with a false rung · case 04

After a restart the off-delay output is on for a full preset even though its rung was never true.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Off-delay timer (TOF) evaluated once per scan; scans is a list of [rung_in, elapsed_ms]. At power-up the timer is expired (ACC = preset, DN false). A true rung clears ACC and sets DN. After the rung drops, ACC accumulates each later false scan's elapsed time (the true-to-false scan itself is not credited), clamped at preset, and DN clears when ACC >= preset. TT is DN and not rung_in. Return [DN, TT, ACC] 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 11	`[[true, true, 20], [true, false, 0], [true, true, 0], [true, true, 20], [true, true, 30], [true, true, 60], [true, false, 0]]`	`[[false, false, 100], [true, false, 0], [true, true, 0], [true, true, 20], [true, true, 30], [true, true, 60], [true, false, 0]]`	Failed

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