FA-92027 / PLC ladder logic scan cycles / Member archive

Start button overrides stop and trip in the same scan · case 02

Holding start while pressing stop keeps the motor running.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Three-wire motor starter rung evaluated per scan. Each scan is a dict with optional keys start (pushbutton, default false), stop_nc (normally-closed stop contact: true means not pressed, default true), ol_nc (normally-closed overload contact, default true), reset (default false) and power (default true). An open overload contact latches a trip (retentive); reset clears it only on a scan where the overload contact is closed. The motor runs on start or its own seal-in, and only while stop_nc is closed and no trip is latched; stop and trip dominate start. A power-loss scan drops the motor and requires the start button to be seen released before it can start again. Return [motor, tripped] 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 while overload open	`[[true, false], [false, true], [true, true]]`	`[[true, false], [false, true], [false, true]]`	Failed

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