AdvancedStep~20 min

Step Sequence

A two-step batch cycle driven by step coils: press Start → FILL until the high level, then DRAIN until the low level, then done. This is how real machine sequences are structured one coil per step, with clean hand-offs.

Your goal

  • Pressing StartBtn enters Step 1: FillValve ON.
  • When LevelHigh turns ON, hand off to Step 2: FillValve OFF, DrainValve ON.
  • When LevelLow turns ON again, the cycle ends: both valves OFF.
  • The cycle must be repeatable a new StartBtn press runs it again.
Stuck? Show hints (4)
  1. Use latches for the steps: R1: NO StartBtn → OTL Step1.
  2. Hand-off: R2: NO Step1 AND NO LevelHigh → OTL Step2. Then R3: NO Step2 → OTU Step1. Rung order makes the hand-off clean.
  3. End: R4: NO Step2 AND NO LevelLow → OTU Step2.
  4. Keep outputs separate from sequencing: R5: NO Step1 → OTE FillValve. R6: NO Step2 → OTE DrainValve.

Pattern from Contact & Coil's classic ladder logic series.

I/O for this exercise

Inputs
StartBtnCycle start button
LevelHighHigh level sensor
LevelLowLow level sensor
Outputs
FillValveFill valve
DrainValveDrain valve
Internal coils
Step1Step 1 active (filling)
Step2Step 2 active (draining)
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