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Mission 06 · Free ladder logic exercise

Review the AI: Stop priority

Two writes look right. Check which one wins.

An assistant wrote this. It looks right. Test it on the rig before it goes to site. Press Test solution before editing. Press Start once and the pump keeps running after you let go. This needs a , where PumpOn remembers it was started. Pressing Stop or losing Permit turns PumpOn OFF. Stop always wins, even with Start held.

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The 3D pump skid this mission runs on
Test replay

Pump keeps running after a short Start press

Test 1 of 6
Set by the testChecked by the testStep 2 of 2 · 120 ms
Level
Applied
Time
About 8 min
Reward
+150 XP
Rig
3D pump skid
Siemens editors
SCL, LAD
Rockwell editors
Structured Text, Ladder
Course stage
3. Remember a Start request
Automatic tests

What the tests check

  1. A short press of Start keeps PumpOn ON after you let go.
  2. Stop turns PumpOn OFF, even while Start is held.
  3. Turning Permit OFF stops the pump. With Start released, turning Permit back ON does not restart it.
  1. Try it Use the buttons below the model and watch the rig respond.
  2. Test solution Every check runs on your code in one press.
  3. Replay If one fails, step through it one scan at a time to see why.
Why it matters

Where you’d see this on site

Before testing a building's water pump, an engineer checks pump code written by an AI assistant. They hold Start and Stop together.

If Start beats Stop, the pump can keep pushing water into a leaking pipe. Pressing both buttons together catches the mistake before it reaches site.

Where you'd find thisWater for the top floors

In a block of flats, a basement tank holds water from the mains. Two pumps push it up to the taps above. One runs while the other waits as a spare. A low-water switch stops the pumps before the tank runs dry.

  1. Street mainsWater enters the building
  2. Storage tankHolds a supply for the pumps
  3. Water pumpsPush water to the floors aboveYou are here
  4. Taps upstairsResidents use the water they need
On a real site: places, parts and safety
  • Block of flats — pushing water up to the top floors
  • Brewery — sending hot water to a brewing tank
  • Water-treatment works — moving water between filter tanks
  • Office building — pumping water round the heating pipes

A low-water switch can take away permission to run. The pump must stop before it runs dry and damages its seal.

The PLC can say ON even when a pump has failed. Check the measured flow as well as the command.

The Stop input in these exercises is not an emergency stop. Real machines also need guards, emergency stops and a safe way to cut the power.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

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