TOF Timer Still On After the Input Turns Off? That May Be the Point
An off-delay timer intentionally keeps its output condition active for a period after the enabling condition disappears. Confusing that behavior with an on-delay timer can make correct logic look broken. Start with a timing sketch showing the input, timer state, and actual machine command.
Logix TOF teaching example; startup and safety behavior are not universal.
What is happening?
In the referenced Logix TOF behavior, the done state is active while the rung is true and remains active through the off-delay interval after a true-to-false transition. Returning the rung to true affects the timing sequence. Startup and prescan behavior require the instruction documentation; do not extrapolate from a steady-running example.
Check these five things
-
Identify which timer member the downstream logic uses. Enable, timing, and done bits represent different conditions.
-
Draw the intended off-delay interval and label the event that starts it. Confirm the process actually needs delayed removal.
-
Check whether the enabling condition returns before the interval finishes. Repeated true states can prevent the expected delayed-off event.
-
Inspect other assignments and interlocks affecting the final output. A correct timer bit does not prove the load command follows it.
-
Test startup, normal transitions, and mode changes in an isolated environment using the documented platform behavior.
Worked example
For a teaching example with a three-second off delay, the input turns false at time zero and stays false. The timed output condition remains true during the delay and then becomes false. If the input returns true halfway through, the expected timeline changes; that is not evidence of a defective timer.
Quick diagnostic reference
| Question | What to inspect |
|---|---|
| Why is it still on? | Off-delay interval and chosen timer bit |
| Why does it never switch off? | Input retriggering and competing commands |
| Why is startup different? | Prescan and initialization semantics |
The mistake to avoid
Do not place an ordinary off-delay in a safety stop path as a shortcut. Safety-related timing requires an appropriately designed and validated function.
How to verify the fix
Record a timing trace for each intended transition and confirm the final command, not only the timer status, behaves as specified.
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 Off Delay (TOF). The examples and diagnostic tables above are original teaching material; they do not replace the product manual.