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ONS — One Shot

storageBit

[ONS]

Makes a rung true for exactly one scan on each rising edge of its input. Requires a dedicated storage bit per instance.

Operands​

NameType
storageBitBOOL

How It Works​

ONS watches the rung condition that feeds it. The first scan the condition is true, ONS also passes true. On every subsequent scan — while the input stays true — ONS passes false. When the input drops back to false and then goes true again, ONS fires once more.

Input  _____|‾‾‾‾‾‾‾‾|________|‾‾‾‾‾‾|___
ONS    _____|‾|_______________|‾|________

Example — Counting Button Presses With ADD​

Without ONS, ADD runs every scan the button is held, so Count races up many times per press:

XIC(Button)ADD(Count,1,Count)

Add ONS so the ADD rung is true for exactly one scan per press:

XIC(Button)ONS(ButtonPulse)ADD(Count,1,Count)

(A CTU would do the same job with built-in edge detection — no ONS needed. Reach for ONS when the instruction you're driving has no internal edge detection of its own.)

Common Mistakes​

  • Reusing the same storage bit for multiple ONS instructions — each ONS needs its own storage tag.