Machine Skips Several Steps in One Scan? Inspect State Assignment Order
A sequence may advance through several states during one task execution if later logic sees earlier state assignments immediately. When intermediate actions disappear, inspect program structure before adding delays.
Original program-order example; branch evaluation semantics are platform-specific.
What is happening?
Independent conditions evaluated in source order can cascade. Other structures select one branch using the value observed at dispatch. Neither behavior is universally correct; the design must specify whether multiple transitions per execution are allowed.
Check these five things
-
Trace state reads and writes in execution order.
-
Identify actions required in every intermediate state.
-
Compare independent IF logic with branch-selection semantics.
-
Define a next-state or equivalent reviewed transition policy.
-
Test conditions that make consecutive transitions immediately true.
Worked example
A teaching program changes state 10 to 20, then a later IF sees state 20 and changes it to 30 in the same call. A diagnostic screen may never display 20. If an entry action is required there, its execution must be proved rather than inferred from the state names.
Quick diagnostic reference
| Observation | Question |
|---|---|
| Intermediate state invisible | Short-lived or never entered? |
| Action missing | Execution order? |
| Multiple writes | Intentional cascade? |
The mistake to avoid
Adding arbitrary dwell timers can conceal unclear transition semantics. Define the intended execution model and preserve required actions explicitly.
How to verify the fix
Use an internal transition trace in simulation to record every change and action. Confirm the observed sequence matches the approved model under fast and slow feedback conditions.
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.