What ONS Does
The ONS (One Shot) instruction in RSLogix 5000/Studio 5000 executes the output logic for exactly one scan when the input conditions transition from false to true. After that single execution, the output goes false and stays false — even if the input conditions remain true.
The “Energized” Confusion
When you see the ONS instruction highlighted (energized) in the online program, it means it has already executed on this scan. It does not mean it is continuously active. By the next scan, the one-shot storage bit latches and the instruction will not fire again until the input conditions go false and then true again.
Why Animation Misleads You
The RSLogix online animation updates are not fast enough to reliably show a one-scan event. If the ONS fires and completes within a single scan (which it always does), you are unlikely to see the output highlight flash green. The program scan may be 5-20ms, and the screen refresh rate is much slower than that.
Do Not Rely On:
- The green highlight on the ONS output rung — you will likely never see it.
- The state of the output tag in the tag monitor — by the time the display refreshes, the one-shot has already cleared.
What to Do Instead
- Add a test latch. Temporarily add an OTL (Output Latch) instruction after the ONS output. When the one-shot fires, the latch captures it and holds. You can then verify it fired and manually unlatch it.
- Use a counter. Place a CTU (Count Up) after the ONS. Each time it fires, the counter increments. This tells you not just if it fired, but how many times.
- Check the storage bit. The ONS uses a BOOL storage bit. If the storage bit is set (1), the one-shot has already fired and is waiting for the input to go false before it can re-trigger.
Resetting the ONS
The ONS will not fire again until its input conditions go false and then return to true. If your conditions are latched on, the ONS is done — it will not trigger again until those conditions cycle. If you need it to re-fire, you need to reset the input conditions or unlatch the storage bit.