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PLC programming challenges
Challenge 18 · Make it safe · Free

Close the guard, stay stopped

Opening the guard stops the belt. Closing it does not start it.

Start and Stop already work. Add the guard door . GuardClosed is ON while the guard is shut. Opening the guard stops the belt at once. Shutting it again must not restart the belt: the operator presses Start. Start does nothing while the guard is open.

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The 3D conveyor box sorter this challenge runs on
Test replay

Opening the guard stops the belt at once

Test 1 of 4
Set by the testChecked by the testStep 4 of 4 · 240 ms
Level
Easy · 10 points
Kind
Make it safe
Time
About 8 min
Pass rate
New
Machine
3D conveyor box sorter
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
Automatic checks

What the checks test

  1. Opening the guard stops Conveyor on the same .
  2. Shutting the guard does not restart the belt.
  3. Start does nothing while the guard is open.
  4. Stop still stops the belt.

The checks run 4 tests on the machine, scan by scan: Opening the guard stops the belt at once; Shutting the guard does not restart it; Start does nothing with the guard open; Stop still stops it.

These challenges teach the logic. On a real machine, safety functions run on a safety-rated PLC, such as a Siemens F-CPU or a Rockwell GuardLogix, and are designed to ISO 13849.

On site

Where you’d find this

A conveyor has a hinged guard over the transfer point. An interlock switch tells the PLC whether the guard is shut.

Someone shutting a guard from the inside must never start the belt. Starting is always a separate, deliberate action.

The machine

How it should behave

Guard
GuardClosed ON while the guard door is shut.
Open
Stops the belt at once.
Shut
Never restarts the belt.

Press Run and Start. Switch GuardClosed OFF: the belt stops. Switch it ON again: it stays stopped until Start.

Keep going

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