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Mission 64 · Pro PLC programming exercise

Two channels must agree

Read the E-stop and the gate on two channels each, and lock the cell out when a pair disagrees.

Stop on either channel of the E-stop or the gate. If the two channels of one device disagree for 0.5 s, a fault that only clears once both channels have opened together.

Training only. This is not a certified safety system. On a real machine these rules run in a safety relay or a safety PLC, designed and validated to ISO 13849-1 or IEC 62061.

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The 3D robot safety cell this mission runs on Pro mission
Test replay

Both channels, used properly

Test 1 of 5
Set by the testChecked by the testStep 19 of 19 · 2 s
Level
Advanced
Time
About 40 min
Reward
+450 XP
Rig
3D robot safety cell
Siemens editors
Ladder (LAD), SCL
Rockwell editors
Ladder, Structured Text
Beckhoff editors
Structured Text
Course stage
33. Safety lab 2: trust nothing single
Automatic tests

What the tests check

  1. Either channel of the E-stop or the gate stops the robot at once.
  2. Channels that disagree for 0.5 s latch a fault and light FaultLamp.
  3. A short difference, under 0.5 s, is not a fault.
  4. A fault survives both channels coming back; it clears only when both are OFF together.
  5. No reset while a fault is latched.
  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

Safety relays read E-stops and gate switches on two channels. A stuck contact shows up as a pair that disagrees.

With one channel, a welded contact or a short in the cable can hide a pressed E-stop. The second channel catches it, and the latch stops anyone resetting around it.

Where you'd find thisKeep people out while the arm moves, and prove it every time

A robot stacks cartons inside a fence. A gate lets a fitter in, a light curtain lets the cartons in, and an emergency stop sits outside. Two contactors in a row cut the robot's power, so one that sticks cannot keep it running.

  1. InfeedCartons through the curtain
  2. PickRobot takes the carton
  3. GuardStop, reset, restartYou are here
  4. PalletSix cartons, then change
On a real site: places, parts and safety
  • Robot palletising and packing cells
  • Machine tending: robots loading presses and machine tools
  • Any guarded machine with a door, a curtain or an E-stop

Training only: this rig is not a certified safety system. Real safety functions run in safety relays or a safety PLC, designed and validated to ISO 13849-1 or IEC 62061.

IEC 60204-1 sets out stop categories 0, 1 and 2 and how an emergency stop must behave. Which category a machine uses comes from its risk assessment.

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

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