FA-92100 / PLC ladder logic scan cycles / Member archive

Sequencer advances every scan while enable is held · case 05

Holding the step input races through the whole sequence.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Sequencer output (SQO). words is the sequencer file whose element 0 is the home step; position starts at 0. On each rising edge of enable the position advances; past the last element it wraps to 1 (not 0). Each advance writes dest = (dest AND NOT mask) OR (word AND mask) in 16 bits. A reset input returns position to 0 without writing dest and has priority over a coincident advance; the enable edge memory is always updated. steps is a list of [enable, reset]; return [position, dest] 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 6	`[[0, 57202], [1, 54647], [2, 54647], [0, 54647], [0, 54647], [0, 54647]]`	`[[0, 57202], [1, 54647], [1, 54647], [0, 54647], [0, 54647], [0, 54647]]`	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.