Fault 08 — Intermittent Bug
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Briefing
A motor system works normally in most cases but occasionally refuses to restart after a stop command. This only happens when the operator presses Start quickly after Stop — while the shutdown sequence is still running. Use slow-scan step mode to catch the race condition.
Objectives
- Reproduce the failure: press START while SHUTDOWN_ACTIVE is still true
- Use slow-scan step mode to observe both SET and RESET rungs firing
- Identify which rung wins when both fire simultaneously
- Add the interlock condition to prevent START during shutdown
Hints
- Switch to Step scan mode and slow things down
- Trigger SHUTDOWN_ACTIVE (via STOP_PB), then immediately press START before shutdown completes
- Watch the RUN_BIT state after the scan where both conditions are simultaneously true
I/O Table
Inputs
START_PBStart push-button
BOOL · %I0.0
STOP_PBStop push-button (NC)
BOOL · %I0.1
Outputs
MOTOR_CONTACTORMotor contactor coil
BOOL · %Q0.0
SHUTDOWN_LAMPShutdown-in-progress lamp
BOOL · %Q0.1
Your program will be tested against:
All test cases run automatically when you submit. Assertions are hidden until you pass.
- #1Normal start and stop
Motor starts and stops normally
- #2Motor starts on rapid restart (during shutdown)
START_PB pressed while shutdown is in progress — motor should start
Related scenarios
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