Excellent PLC
Chapter 5: Timer Module Diagnostics
5.1 Symptom Description
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Timer output triggers earlier or later than configured
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No module fault LED illuminated
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PLC reports sequence anomaly
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Field wiring verified correct
Example: Configured delay 2s → measured delay 3.8s
5.2 Required Tools
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Engineering programming station
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Digital multimeter (DC 24V, mV precision)
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Oscilloscope (≥10 MHz bandwidth)
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Torque screwdriver for terminal check
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Spare F2103a module (for replacement verification)
5.3 Step-by-Step Diagnostic Procedure
Step 1: Configuration Verification
2. Read timer preset value.
3. Compare with project archive.
4. Confirm RUN mode.
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Expected: Preset value matches design specification
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Observed deviation indicates either corrupted memory or internal drift
Step 2: Output Verification
2. Trigger timer input.
3. Measure output terminal voltage with DMM.
4. Record activation timing.
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If output triggers correctly: external load wiring issue suspected
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If output triggers incorrectly: internal timing logic or supply issue suspected
Step 3: Power Supply Assessment
2. Observe for dips during full cabinet load.
3. Verify ripple < 150 mV peak-to-peak.
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Supply dips or spikes > ±10% can corrupt counter logic
Step 4: Oscilloscope Timing Verification
2. Probe output terminal (CH2).
3. Capture at 1 kHz repetition.
4. Measure actual delay per cycle.
5. Repeat for 20 cycles.
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Calculate mean delay and standard deviation
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Compare with configured value
5.4 Fault Identification
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Internal oscillator drift → delays longer than expected
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Power sag or ripple → counter register resets, causing early trigger
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Memory corruption → configuration CRC mismatch
5.5 Corrective Action
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Replace F2103a module if timing drift > ±5% of specification
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Verify PSU stability
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Re-download timer configuration
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Perform 50-cycle validation test
– Delay within ±2% of nominal
– Output LED functions correctly
– Safety PLC sequence restored
5.6 Preventive Maintenance
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Annual timing verification under full load
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Monitor supply voltage dips during operation
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Maintain spare timer modules for critical circuits
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Avoid environmental extremes (temperature > 60°C)
5.7 Notes
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Timer drift failures are subtle; they appear as functional but unsafe behavior
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Always measure real-world timing with oscilloscope before assuming module healthy
Conclusion:
Following this structured manual procedure ensures correct diagnosis and repair of Black Horse F2103a Timer Module timing faults in Planar F safety applications.