Practice missions
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Practice missions
Mission 05 · Free PLC fault-finding exercise

Stop doesn't stop it

Find the path that gets round Stop.

Find and fix the path that gets round Stop. With Permit ON, a press of Start must PumpOn, so it stays on after Start is let go. Stop, or losing Permit, must break every path that holds it on. Putting Permit back must not restart the pump without a new Start.

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The 3D pump skid this mission runs on
Test replay

Operator report: Stop after Start is released

Test 1 of 7
Set by the testChecked by the testStep 5 of 5 · 440 ms
Level
Applied
Time
About 12 min
Reward
+200 XP
Rig
3D pump skid
Siemens editors
SCL, LAD
Rockwell editors
Structured Text, Ladder
Course stage
3. Remember a Start request
The operator’s report · pump skid
P-101 started up fine. I let go of Start, then hit Stop, and it just kept running. The only way I could stop it was to switch the Permit off.
Automatic tests

What the tests check

  1. The pump keeps running after Start is let go.
  2. Stop wins on every , even with Start held.
  3. Losing Permit stops the pump. Putting it back alone 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

A caretaker presses Stop on a running basement water pump. The motor keeps turning and the flow meter still shows water moving.

The operator cannot stop the water while the code that keeps the pump running ignores Stop. A leaking pipe can keep flooding the room.

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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