Why PLC One-Shot Logic Needs a PLS Pulse

When an input remains ON, the PLC keeps executing the same rung every scan. One-shot behavior needs an edge pulse.

Why one button press can execute many times

The PLC executes the same logic every scan while an input stays ON. This rung therefore adds 5 on every scan:

|----[ X0 ]----[ ADD D0 K5 D0 ]----|

Use a one-scan rising-edge pulse

|----[ X0 ]------[ PLS M100 ]---------|
|----[ M100 ]----[ ADD D0 K10 D0 ]----|

M100 is ON for one scan only when X0 changes from OFF to ON.

Scan example

Typical uses

• One-step HMI/button increment
• Step-transition events
• Single-shot MOV storage
• Preventing repeated counter events