Seal-In Circuit Won\’t Stop? Check Where the Stop Condition Actually Sits
A seal-in branch should maintain an ordinary run request only while the required enabling conditions remain valid. If a holding branch bypasses the stop condition, the logic can keep the request true after the operator expects it to clear. Draw the Boolean expression, not just the familiar ladder shape.
Original Boolean teaching example, not production-ready safety logic.
What is happening?
A simple teaching expression is RunNext = StopPermissive AND (StartRequest OR RunPrevious). Placing the stop condition outside the entire holding branch makes its role explicit. This is ordinary control logic only; a safety stop function cannot be implemented or validated merely by copying this expression.
Check these five things
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Define the meaning of the stop-related bit. StopPressed and StopPermissive have opposite intended meanings and should not be confused.
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Translate the rung into a Boolean expression and check which branches the stop condition actually governs.
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Cross-reference the run request for latches, unlatches, or other writers that may preserve it elsewhere.
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Test simultaneous start and stop requests and document which must take priority under the design.
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Test startup and mode transitions in an isolated simulator before any machine change is approved.
Worked example
For the teaching expression, StopPermissive = 0 makes RunNext = 0 regardless of StartRequest or RunPrevious. If the program instead uses (StopPermissive AND StartRequest) OR RunPrevious, the previous run state bypasses that condition. The parentheses expose the defect clearly.
Quick diagnostic reference
| Stop permissive | Start or previous run | Teaching result |
|---|---|---|
| 0 | Either value | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
The mistake to avoid
Do not describe an ordinary PLC seal-in circuit as an emergency-stop system. Safety-related stopping requires the appropriate architecture and validation.
How to verify the fix
Use a truth table to confirm priority, then verify the approved machine behavior through the required commissioning procedure.
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: Boolean and output instruction reference. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.