Excellent PLC
By Robert Sinclair – Industrial Electronics Reliability Engineer
The most expensive failures are not the ones that break equipment.
They are the ones that quietly erode safety margins.
This case involved a Honeywell 07191/1/1 RS485 communication board that survived a nearby lightning event. Power stayed on. Communication stayed up. No alarms were generated.
Everyone moved on.
They shouldn’t have.
What Happened Before the Symptoms Appeared
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Severe thunderstorm
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No direct strike on the control building
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Multiple inductive loads tripped and recovered
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RS485 network resumed operation automatically
From a system perspective, everything looked normal.
Weeks later, communication issues began to surface.
Early Symptoms (Often Ignored)
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Occasional checksum retries
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Slight increase in response time
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One device occasionally missing a poll
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No consistent fault pattern
Nothing dramatic enough to justify replacing hardware.
That’s exactly how latent damage hides.
Why Surge Damage Is Different
A surge doesn’t need to destroy a component to damage it.
In RS485 transceivers, surge stress can cause:
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Input protection diode leakage
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Reduced output drive strength
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Slower edge transitions
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Increased susceptibility to noise
The 07191/1/1 board was still functional — just operating with reduced electrical margins.
What the Bus Looked Like Electrically
Measured at the far end of the network:
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Differential voltage lower than historical baseline
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Signal rise/fall times noticeably slower
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Noise immunity reduced under load
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Reflections more pronounced
No single parameter violated limits.
All margins were thinner.
Why This Is Hard to Diagnose
Standard checks passed:
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Continuity OK
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Termination OK
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Grounding OK
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Configuration unchanged
But RS485 doesn’t fail at a threshold.
It fails statistically.
Errors increase gradually until the system becomes unreliable.
The Telltale Comparison
We temporarily replaced the 07191/1/1 board with an identical spare.
Same wiring.
Same network.
Same configuration.
Result:
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Signal amplitude increased
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Edge transitions sharpened
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Error rate dropped to zero
That confirmed it: the original board had suffered soft damage.
Corrective Strategy
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Permanently replaced the degraded communication board
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Installed additional surge protection on RS485 lines
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Improved cable shielding at building entry points
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Added post-storm communication quality checks
Recommended monitoring logic:
Don’t wait for total failure. Watch the trend.
Key Engineering Lessons
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Surges don’t have to kill hardware to damage it
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Communication boards can degrade invisibly
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Error rate trends matter more than alarms
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“Still online” does not mean “still healthy”
Final Thought
The Honeywell 07191/1/1 RS485 communication board survived the surge.
But survival is not the same as full recovery.
In communication systems, lost margin is borrowed time — and it always comes due.
— Robert Sinclair