Your PLC TON Timer Keeps Resetting? Look for the One-Scan Dropout
A timer that never finishes may be doing exactly what its instruction requires. A brief loss of its enabling condition can reset a nonretentive on-delay timer before the online display makes the interruption obvious. Trend the condition and inspect the instruction's execution before increasing the preset.
Logix TON is the reference example; verify other vendors' timer semantics separately.
What is happening?
For the documented Logix TON instruction, a false rung condition clears the nonretentive timing state. Other platforms have their own instruction and call semantics. A timer that is not executed is not necessarily equivalent to one executed with a false input, so conditional routine calls deserve separate attention.
Check these five things
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Confirm the actual instruction type, preset units, controller family, and language. Do not assume a TON, TONR, and RTO have interchangeable behavior.
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Observe the complete enabling expression, not just the start button. Include permissives, mode bits, and intermediate conditions.
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Capture short dropouts with an appropriate controller trace or diagnostic latch in an approved test environment. An HMI refresh may miss them.
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Cross-reference the timer instance and its members. Look for another instruction, reset, initialization, or assignment changing the same storage.
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Confirm the routine executes at the intended interval. Review conditional calls and task scheduling before deciding the timer itself is faulty.
Worked example
A simulated five-second TON is enabled for 4.8 seconds, loses its input for one execution, then becomes true again. A nonretentive timer starts a new interval, so it can appear stuck below five seconds. The right question is why the enabling condition drops, not whether the preset should be shortened.
Quick diagnostic reference
| Evidence | Likely investigation |
|---|---|
| Accumulator returns to zero | False condition or explicit reset/write |
| Accumulator stops changing | Execution path or task behavior |
| Done bit true; action absent | Downstream logic and command ownership |
The mistake to avoid
Replacing a TON with a retentive timer can change the process requirement. Do that only when accumulated time across interruptions is actually intended and reviewed.
How to verify the fix
Test an uninterrupted interval, a deliberate interruption, and a restart in simulation. Confirm the observed timing matches the documented instruction and the process requirement.
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: Logix Timer On Delay (TON). The examples and diagnostic tables above are original teaching material; they do not replace the product manual.