FA-91907 / PLC ladder logic scan cycles / Member archive

RES clears the RTO accumulator but leaves DN latched · case 02

After a reset the done bit stays on while ACC reads zero.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Retentive timer (RTO) on rung 1 and its reset (RES) on a later rung, evaluated per scan; scans is a list of [enable, reset, elapsed_ms]. An enabled scan whose previous scan was also enabled adds the elapsed time (clamped at preset); DN sets when ACC >= preset while enabled. A disabled rung retains ACC and DN. Because RES is a later rung, a reset in a scan clears ACC and DN after the RTO has run; the enable edge memory is not touched by RES. Return [DN, ACC] 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: reset done timer while disabled	`[[false, 0], [true, 50], [true, 50], [true, 0], [true, 0]]`	`[[false, 0], [true, 50], [true, 50], [false, 0], [false, 0]]`	Failed

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