Ladder
Left rail is logic power. Green means it solved true on the last scan.
Truth table for this program
How a scan works
- Inputs are the switches in the I/O panel. Forcing one changes its bit in the input image table.
- Rungs solve top to bottom, left to right. A normally open contact (XIC) conducts when its bit is 1. A normally closed contact (XIO) conducts when its bit is 0.
- OTE writes the rung result every scan. OTL sets a bit and holds it. OTU clears it while the rung is true. Later rungs win when two write the same bit.
- Seal-in: NC Stop in series, then NO Start in parallel with a NO of the output, then an OTE of that output.
- TON (on-delay) times while its rung is true and sets DN when ACC reaches PRE; it resets when the rung goes false. TOF (off-delay) keeps DN on for PRE after its rung goes false. Each scan is 0.1 s of PLC time.
- CTU adds one each time its rung goes from false to true; DN sets when ACC reaches PRE. RES zeros a timer or counter.
- Use Next rung to watch one rung solve at a time, and the truth table to compare your logic with the exercise.
Keyboard shortcuts
- Space run / stop the scan (when nothing is focused)
- S single scan · R next rung
- 1–8 toggle inputs I0–I7
- Ctrl+Z undo · Delete removes the focused element
- Arrow keys move through the instruction palette