Two Devices Share One Timer State? Check the Function Block Instance
Two calls to one reusable definition do not automatically represent independent devices. The instance identifies stored state. If separate motors influence each other inside otherwise identical logic, inspect the instance data used by each call.
Stateful block example; instance and parameter semantics depend on the platform.
What is happening?
Reusable blocks may retain timers, previous input states, alarms, and sequence information. Sharing a definition is normal; unintentionally sharing per-device state is different. Add-On Instructions and IEC function blocks implement instances through platform-specific mechanisms.
Check these five things
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List each call and the physical asset it represents.
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Compare instance tags or data blocks, not only definition names.
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Inspect local state in the context of the failing instance.
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Check global tags and reference parameters for additional shared writable storage.
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Test devices with deliberately different timing and fault conditions in simulation.
Worked example
Two simulated conveyors call one motor-control definition using the same instance. Conveyor A starts a feedback timer; the call for conveyor B resets that stored timer. Independent instance state addresses the conflict, but external shared tags also need review.
Quick diagnostic reference
| Item | Typical ownership |
|---|---|
| Block definition | Reusable across devices |
| Feedback timer | Per device |
| Coordination request | Explicitly shared if required |
The mistake to avoid
Copying a rung and changing its output address can leave the instance tag unchanged. A separate visible rung does not prove that the underlying memory is separate.
How to verify the fix
Exercise simultaneous starts, unequal feedback delays, and a fault on only one device. Confirm that unrelated state remains unchanged and that intentional coordination still works.
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: Add-On Instructions. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.