PLC Resends a Command Every Scan? Separate State Entry From State Activity
An action intended once on entering a state can accidentally run throughout that state. If a device keeps receiving new requests, inspect whether the program distinguishes entry, continuous activity, and exit.
Original sequence architecture; SFC action semantics depend on the implementation.
What is happening?
Stateful sequences often need different timing for each kind of action. A maintained state condition is not an entry event. The design must also define re-entry after interruptions and what happens if a request has not yet been accepted.
Check these five things
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List actions that should happen once and those that must remain active.
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Inspect how previous and current state are recorded.
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Trace transitions occurring more than once in a task execution.
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Simulate a state lasting many cycles.
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Test interruption and deliberate re-entry separately.
Worked example
A teaching state starts a file transfer. Calling the request logic on every scan repeatedly restarts the transfer before it finishes. A defined entry request followed by a monitored completion phase preserves the distinction between issuing work and waiting for it.
Quick diagnostic reference
| Action | Timing |
|---|---|
| Entry | Once per accepted entry |
| Activity | While state remains active |
| Exit | According to transition contract |
The mistake to avoid
Adding a generic one-shot without reviewing re-entry can suppress a legitimate retry. The event must represent the intended state lifecycle, not merely a convenient bit edge.
How to verify the fix
Count issued and accepted requests through normal operation, long waits, and recovery. Confirm exactly the intended number of actions occurs for each state entry.
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: Sequential Function Charts. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.