FA-91965 / PLC ladder logic scan cycles / Member archive

XIO on a never-written tag evaluates false · case 05

A rung built on an examine-if-open contact stays off until the referenced bit has been written once.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Ladder program scan. rungs is a list of [coil, branches]; a branch is a series list of [XIC|XIO, tag] contacts and the rung is true if any branch is fully true. Each scan item is [start_changes, mid_changes]: start_changes [[tag, value]] update physical inputs before the input scan; the input image is then frozen for the program scan. mid_changes [[rung_index, tag, value]] change physical inputs just before that rung executes and are only seen by the next input scan. Non-input tags are read from the data table, where earlier rungs of the same scan have already written their coils (last OTE wins). Missing tags read false. Return the sorted energized O: tags after each 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 7	`[[], [], [], []]`	`[["O:1"], ["O:1"], ["O:1"], ["O:1"]]`	Failed

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