PLC State Number Is Invalid? Your CASE Needs a Defined Fallback
An unexpected state value should produce a defined response. If a sequence stops updating with no useful alarm, inspect the unmatched path of its CASE statement before adding another normal transition.
Original state-machine example; the machine design defines its fault response.
What is happening?
CASE selects the branch matching its expression. An optional fallback handles unmatched values, but the application must decide what that means operationally. Silently retaining previous commands is not an adequate assumption for every machine.
Check these five things
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List all valid state values and their intended initialization.
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Trace every writer of the state variable.
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Inspect the unmatched branch and output ownership.
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Simulate invalid values outside production.
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Define recovery through the approved sequence design.
Worked example
A teaching sequence supports states 0, 10, and 20. A malformed recipe supplies 15. None of the normal branches runs, and a previous request remains stored. A reviewed fallback can identify the invalid value and request the defined fault response without pretending the cycle completed.
Quick diagnostic reference
| Value | Expected interpretation |
|---|---|
| Known state | Defined branch |
| Unknown value | Explicit diagnostic |
| Recovery request | Validated restart path |
The mistake to avoid
Automatically assigning zero can trigger startup actions or motion. The fallback must reflect the machine requirements rather than treating any known number as safe.
How to verify the fix
Test every supported state, invalid values, and the authorized recovery path. Record which outputs remain controlled during the fault and confirm that invalid state data cannot disappear without evidence.
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
CODESYS: CASE statement. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.