FA-92177 / PLC ladder logic scan cycles / Member archive

Unloading an empty FIFO overwrites the destination · case 02

An unload request on an empty queue changes the destination word.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

FIFO load (FFL) and unload (FFU) pair on one file of the given length. ops is a list of [op, value], one triggered instruction per scan. Load stores value at the position and increments it; when full (position == length) the load is ignored. Unload moves element 0 to dest, shifts the file toward 0 and zeroes the vacated last element; unloading an empty FIFO leaves dest unchanged. DN is position >= length and EM is position == 0. Return [position, dest, DN, EM, file] 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 1	`[[0, 0, false, true, [0]], [1, 0, true, false, [18]], [0, 18, false, true, [0]], [1, 18, true, false, [68]], [1, 18, true, false, [68]], [0, 68, false, true, [0]], [0, 0, false, true, [0]], [0, 0, false, true, [0]], [1, 0, true, false, [94]], [0, 94, false, true, [0]]]`	`[[0, 0, false, true, [0]], [1, 0, true, false, [18]], [0, 18, false, true, [0]], [1, 18, true, false, [68]], [1, 18, true, false, [68]], [0, 68, false, true, [0]], [0, 68, false, true, [0]], [0, 68, false, true, [0]], [1, 68, true, false, [94]], [0, 94, false, true, [0]]]`	Failed

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