Practice missions
Practise in
0 XP
Practice missions
Mission 43 · Pro PLC programming exercise

One car at a time

Raise the arm for a ticket, lower it once the car is clear.

Raise the arm when a waiting car's driver presses for a ticket, show green once it is up, and lower it after the car has driven clear of the safety loop.

This mission is part of ProTry the full course, every mission and the expert challenges free for 7 days. No card needed.
The 3D car park barrier this mission runs on Pro mission
Test replay

A car goes through

Test 1 of 3
Set by the testChecked by the testStep 9 of 9 · 520 ms
Level
Applied
Time
About 20 min
Reward
+300 XP
Rig
3D car park barrier
Siemens editors
SCL, LAD
Rockwell editors
Structured Text, Ladder
Course stage
26. Machine lab 1: real machines, clear rules
Automatic tests

What the tests check

  1. A ticket press only counts with a car on EntryLoop.
  2. RaiseArm runs until ArmUp; LowerArm runs until ArmDown.
  3. GreenLight shows only while the arm is fully up and the car is going through.
  4. The arm lowers only after the car has left SafetyLoop and EntryLoop.
  5. A car on SafetyLoop while lowering sends the arm straight back up.
  6. RaiseArm and LowerArm are never ON together.
  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

Every car park entry runs this sequence, thousands of times a day, usually on a small PLC or a dedicated barrier controller.

An arm that comes down on a car roof damages cars and leads to complaints and claims. The safety loop is there so the logic can prevent it.

Where you'd find thisLet cars in one at a time, without ever hitting one

Hospital, airport and office car parks use barriers like this all day. Loops of wire cut into the road detect the metal of a car. One loop says a car is waiting; the safety loop under the arm stops it coming down on a car that is still passing.

  1. Car arrivesEntry loop
  2. TicketDriver presses
  3. Arm upCar passesYou are here
  4. Arm downLane closed
On a real site: places, parts and safety
  • Car parks and multi-storey garages
  • Factory and depot gatehouses
  • Toll plazas and ferry terminals

Never lower an arm while the safety loop detects a vehicle.

Real barriers add photocells and a pressure edge for pedestrians and cyclists, which loops cannot detect.

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

Keep going

Next in the course