Controls
Scenarios:
Timing diagram
Physical input
PLC input image (sampled at scan boundaries)
PLC output
Scan boundary
What happened?
Pulse / scan ratio
—
Physical pulse duration
—
PLC saw input as high for
—
Try these
- Slow scan, short pulse — set scan = 100 ms, pulse = 10 ms, then drag the pulse start. Watch it flip between caught and missed depending on alignment with a scan boundary.
- Fast scan, same pulse — drop scan to 5 ms with the same 10 ms pulse. Now it's always caught.
- Latching rescue — turn on the latch with a slow scan and a shifted pulse. Even if the pulse is short, the output stays on after the input drops.
- Worst case — the maximum delay between input change and output change is roughly two scan times: one to sample, one to update.