FA-92118 / PLC ladder logic scan cycles / Member archive

Shift register moves tracking bits toward the load end · case 03

Part-present bits travel backwards along the conveyor model.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Bit shift left (BSL) register tracking parts on a conveyor. On each rising edge of the encoder clock the bit at the far end (index length-1) moves to the unload bit, every bit moves one position toward the far end, and bit 0 loads the part-present sensor sampled on that same scan. The unload bit holds its value until the next shift. pulses is a list of [clock, sensor]; return [register string index 0 first, unload] 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: sensor changes on edge scan	`[["000", 0], ["000", 0], ["000", 0], ["001", 0], ["001", 0]]`	`[["000", 0], ["000", 0], ["000", 0], ["100", 0], ["100", 0]]`	Failed

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