FA-92137 / PLC ladder logic scan cycles / Member archive

Step sequencer cascades through several transitions in one scan · case 02

Steps whose transition is already satisfied are skipped without ever being active for a scan.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Cyclic step sequence with one active step. steps is a list of [min_time_ms, transition_tag]. Each scan [dt, inputs] first adds dt to the step time unless the step became active on the previous scan, then fires the transition if step time >= min_time and the transition tag is true. At most one transition fires per scan; after the last step the sequence returns to step 0; the new step time is 0. Step 0 is active before the first scan as if just entered. Return [active_step, step_time, entered_this_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: two zero-time steps	`[[3, 0, true], [3, 0, false], [3, 10, false]]`	`[[1, 0, true], [2, 0, true], [2, 0, false]]`	Failed

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