FA-92084 / PLC ladder logic scan cycles / Member archive

Sequencer wraps to the home element instead of step 1 · case 04

After the last step the machine outputs the home pattern and needs an extra advance before step 1.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Sequencer output (SQO). words is the sequencer file whose element 0 is the home step; position starts at 0. On each rising edge of enable the position advances; past the last element it wraps to 1 (not 0). Each advance writes dest = (dest AND NOT mask) OR (word AND mask) in 16 bits. A reset input returns position to 0 without writing dest and has priority over a coincident advance; the enable edge memory is always updated. steps is a list of [enable, reset]; return [position, dest] 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	`[[1, 51387], [1, 51387], [1, 51387], [1, 51387], [0, 59579], [0, 59579], [0, 59579], [1, 51387]]`	`[[1, 51387], [1, 51387], [1, 51387], [1, 51387], [1, 51387], [1, 51387], [1, 51387], [1, 51387]]`	Failed

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