FA-92253 / PLC ladder logic scan cycles / Member archive

Free-running clock timer mishandles the tick counter rollover · case 03

A delay that spans the 65535-to-0 rollover reports negative elapsed time and never finishes.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Delay timer built on a 16-bit free-running clock register (ticks wrap from 65535 to 0). samples is a list of [enable, clock] read each scan. The first enabled scan captures the start tick; elapsed is (clock - start) modulo 65536. Done latches once elapsed >= preset and stays set while enabled even if the elapsed value later wraps. A disabled scan clears start and done. Return [done, elapsed capped at preset once done] per scan, [False, 0] when disabled.

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 5	`[[false, 0], [true, 5], [false, 0], [false, 0], [false, -50787], [false, -32185], [false, -11567], [true, 5], [true, 5]]`	`[[false, 0], [true, 5], [false, 0], [false, 0], [true, 5], [true, 5], [true, 5], [true, 5], [true, 5]]`	Failed

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