Excellent PLC
Incident Overview
Date: October 29, 2025
Location: Refinery Control Room, Unit 3
Reported by: Senior Control Engineer
A Triconex 3000110-360 Central Processor Unit (CPU) suddenly went into a fault state during normal operation. The system alarm displayed:
“Main Processor Fault — Primary CPU offline”
The incident affected several safety control loops connected to the Triconex TMR (Triple Modular Redundant) system.
1️⃣ Initial Symptoms
When engineers arrived on-site, they observed the following indicators:
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FAULT LED: Solid red
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RUN LED: Off
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COMM LED: Flashing irregularly
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Engineering workstation unable to establish communication via TriStation 1131
Additionally, the redundant CPUs (B and C) remained active, maintaining process safety but triggering a redundancy loss alarm.
2️⃣ Step-by-Step Diagnostic Process
Step 1: Power and Communication Check
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Verified the 24V DC power supply: stable at 24.1V.
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Checked the backplane connection for the CPU: seated properly.
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Ethernet link lights were active — indicating physical connection OK.
Step 2: Module Swap Test
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Moved the suspected faulty 3000110-360 CPU to another slot.
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Fault followed the module → confirmed issue with the processor itself, not the rack or power supply.
Step 3: Software Connection Attempt
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Attempted to connect using TriStation 1131.
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Communication failed — CPU not responding to requests.
Step 4: Visual and Thermal Inspection
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Slight discoloration on the backplane connector edge.
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Surface temperature of the CPU casing measured 58°C, higher than the normal 45°C operating range.
3️⃣ Root Cause Analysis
After removing the module and performing a detailed bench inspection, the following was determined:
| Observation | Possible Cause | Conclusion |
|---|---|---|
| Overheating near the processor chipset | Inadequate ventilation or aging component | Thermal degradation of CPU components |
| Fault LED constant red | Internal watchdog failure | Processor self-test failed |
| Communication lost via TriStation | Embedded firmware corruption | Firmware crash or flash memory damage |
The root cause was identified as firmware corruption triggered by overheating, resulting in a self-check failure during runtime.
4️⃣ Corrective Actions
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Replaced the faulty CPU module (3000110-360) with a verified spare unit.
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Reloaded configuration and firmware via TriStation 1131.
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Performed system synchronization with redundant processors.
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Conducted burn-in testing for 24 hours under normal load.
After replacement, the system stabilized, and redundancy was fully restored.
5️⃣ Preventive Recommendations
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Maintain cabinet temperature below 40°C with active ventilation.
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Periodically back up firmware and configuration files.
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Avoid frequent hot-swapping — always power down before CPU replacement.
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Inspect backplane connectors for dust or oxidation every 6 months.
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Keep at least one spare 3000110-360 CPU in stock for emergency use.
✅ Conclusion
The Triconex 3000110-360 CPU fault was caused by firmware corruption due to thermal stress, leading to a watchdog failure.
Replacement and reconfiguration restored system operation without further data loss.
This case highlights the importance of environmental monitoring, regular backups, and preventive maintenance in high-availability Triconex TMR systems.