PLC Event Buffer Loses the Oldest Fault? Check Full Versus Empty
A circular buffer needs an explicit rule for becoming full. If fault history disappears during a burst, inspect occupancy together with read and write positions. Matching indices alone may mean either full or empty in an incomplete design.
Original buffer example; task scheduling informs the concurrency review.
What is happening?
Circular storage reuses array positions after wrapping. Designs distinguish states by reserving a slot, maintaining a count, or tracking generations. They must also define whether a full buffer rejects new events or overwrites old ones, with visible loss accounting.
Check these five things
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Document capacity, index range, and the overflow policy.
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Identify what distinguishes empty storage from full storage.
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Test wraparound with a small capacity that is easy to inspect.
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Review producer and consumer consistency across tasks.
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Expose dropped or overwritten event counts to maintenance.
Worked example
A four-slot model uses only read and write indices. After four writes, the write index returns to the read index, as it did when empty. An occupancy or generation scheme resolves the ambiguity, but concurrent updates and overflow behavior still require testing.
Quick diagnostic reference
| Test | Required evidence |
|---|---|
| Empty | Unambiguous state |
| Capacity reached | Explicit full state |
| Extra event | Documented loss or rejection |
The mistake to avoid
Increasing array size postpones overflow without correcting state logic. The failure may simply become rarer and harder to reproduce during a machine-wide alarm burst.
How to verify the fix
Pass known event sequences through empty, partially full, full, and wrapped states. Compare order and loss counters with expectations while production and consumption overlap. Preserve the test sequence as a maintenance regression check.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Rockwell Automation: Tasks, Programs, and Routines. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.