FA-92173 / PLC ladder logic scan cycles / Member archive

FIFO shift duplicates the last element into the vacated slot · case 03

Stale data remains visible in the file beyond the valid position.

Member previewVariant 3 · 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, 0]], [1, 0, false, false, [30, 0]], [2, 0, true, false, [30, 44]], [2, 0, true, false, [30, 44]], [1, 30, false, false, [44, 44]], [2, 30, true, false, [44, 20]], [1, 44, false, false, [20, 20]], [2, 44, true, false, [20, 34]], [1, 20, false, false, [34, 34]], [0, 34, false, true, [34, 34]], [1, 34, false, false, [90, 34]], [0, 90, false, true, [34, 34]]]`	`[[0, 0, false, true, [0, 0]], [1, 0, false, false, [30, 0]], [2, 0, true, false, [30, 44]], [2, 0, true, false, [30, 44]], [1, 30, false, false, [44, 0]], [2, 30, true, false, [44, 20]], [1, 44, false, false, [20, 0]], [2, 44, true, false, [20, 34]], [1, 20, false, false, [34, 0]], [0, 34, false, true, [0, 0]], [1, 34, false, false, [90, 0]], [0, 90, false, true, [0, 0]]]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.