FA-92189 / PLC ladder logic scan cycles / Member archive

FIFO empty bit is derived from the unloaded value · case 04

EM reports empty after a zero-valued part is unloaded even though entries remain, or not empty when drained of nonzero parts.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case scope

EM reports empty after a zero-valued part is unloaded even though entries remain, or not empty when drained of nonzero parts.

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: zero value part	`[[1, 0, false, true, [3, 0]], [0, 3, false, false, [0, 0]], [1, 3, false, false, [0, 0]], [0, 0, false, true, [0, 0]], [0, 0, false, true, [0, 0]]]`	`[[1, 0, false, false, [3, 0]], [0, 3, false, true, [0, 0]], [1, 3, false, false, [0, 0]], [0, 0, false, true, [0, 0]], [0, 0, false, true, [0, 0]]]`	Failed

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