PLC Timer Instructions: TON, TOF, RTO With Examples
The PLC timer instructions measures elapsed time inside the PLC program. These timer are used in PLC for timer status to control a motor, fan, valve, alarm, conveyor, or another device.
For example, a conveyor motor may need a five-second start delay. A cooling fan may need to keep running for one minute after a motor stops. A pump may need its total running time recorded for maintenance.
The timer provides the time function. Your ladder logic decides what happens with that information.
What a PLC Timer Actually Does
A PLC timer is a software instruction. It measures time and sets a status bit when the measured time reaches a set value. There are no mechanical contacts to wear out. You change the preset from the HMI without touching any wires.
Every timer has three core parts:
- Preset (PRE or PT) – the target time
- Accumulated value (ACC or ET) – the time already counted
- Status bits – EN (enable), TT (timing), DN or Q (done)
Modern Allen-Bradley Logix controllers count in milliseconds. Siemens TIA Portal uses time literals such as T#5S. Always confirm the time base on the PLC you are using.
A PLC timer measures elapsed time inside the control program. Other ladder logic can then use the timer status to control a motor, fan, valve, alarm, conveyor, or another device.
For example, a conveyor motor may need a five-second start delay. A cooling fan may need to keep running for one minute after a motor stops. A pump may need its total running time recorded for maintenance.
The timer provides the time function. Your ladder logic decides what happens with that information.
How PLC Timers Work During the PLC Scan Cycle
A PLC repeatedly executes its program.
A simplified scan is:
Read inputs → execute program → update outputs → repeat
Timer instructions are evaluated as part of this program execution. The controller updates the timer according to its instruction, enabling condition, preset, and execution environment.
This matters during troubleshooting. A timer does not operate separately from the PLC program. Task execution, scan behavior, and the specific PLC instruction can affect how timing is handled.
For exact behavior, check the documentation for the PLC family you are using.
TON – On-Delay Timer
The TON delays the turn-on of an output. The done bit stays off until the rung has been true for the full preset time.
How it works:
- Rung goes true → EN and TT turn on, ACC starts from zero.
- ACC reaches PRE → DN turns on, TT turns off.
- Rung goes false at any time → everything resets to zero.
If the input drops before the preset is reached, the timer forgets the partial time. That is the key point.
A normal non-retentive TON resets when its enabling rung becomes FALSE. If the timer has a five-second preset and the rung becomes FALSE after three seconds, the timer does not keep those three seconds. The accumulated value returns to zero. When the rung becomes TRUE again, timing starts again.
Real Example: Coal Crusher start delay (power plant)
Large motors draw high current at start. A TON of 4 seconds between the start command and the conveyor load prevents overload trips.
Ladder idea:
Start push-button and overload healthy enable the TON (PRE = 4000).
TON.DN drives the conveyor contactor.
Other common uses
- Sensor debounce (50 to100 ms)
- Alarm delay so short spikes do not trip the system
- Sequence step timeout
- Conveyor start delay when one conveyor must start after another conveyor has reached a safe sequence condition
- Process alarm delay when a sensor condition must remain active for a set period before an alarm is generated
This can help prevent a short signal disturbance from immediately creating an alarm. The actual delay should be based on the process and equipment requirements.
TOF – Off-Delay Timer
The TOF keeps an output on after the input turns off. The done bit is true as soon as the rung is true and stays true for the preset time after the rung goes false.
How it works:
- Rung goes true → DN turns on immediately.
- Rung goes false → TT turns on and ACC starts counting.
- ACC reaches PRE → DN turns off.
If the input returns true before the preset finishes, ACC resets and DN stays on.
What happens when the TOF input goes FALSE? The FALSE transition starts the off-delay timing. The output remains active during the configured delay and then changes state when the preset is reached.
Exact status-bit behavior depends on the PLC instruction and platform, so do not assume that an Allen-Bradley TOF and a Siemens TOF expose identical bits or programming structures.
Real Example – Cooling fan after motor stop (cement plant)
Motor windings stay hot after the motor stops. A TOF keeps the cooling fan running for 180 seconds.
Ladder idea:
Motor run bit enables the TOF (PRE = 180000).
TOF.DN keeps the fan output on.
Other common uses of PLC Timer Instructions
- Lubrication pump purge after conveyor stop
- Door-open alarm that ignores short openings
- Product clear on a conveyor after the photoeye clears
- Motor cooling fan
- Cabinet ventilation
- Conveyor clearing
- Lubrication sequence
- Process air
Why use TOF? It is useful when equipment needs a controlled run-on period.
RTO – Retentive On-Delay Timer
The RTO counts like a TON while the rung is true, but it keeps the accumulated value when the rung goes false. You must use a separate RES instruction to clear it.
How it works:
- Rung true → ACC increases.
- Rung false → ACC holds its value.
- Rung true again → ACC continues from the held value.
- ACC reaches PRE → DN turns on.
- RES instruction clears ACC and DN.
An RTO is used when accumulated time must remain available after the enabling rung becomes FALSE.
Consider a pump that runs for 20 minutes, stops, then runs for another 15 minutes. With an RTO, the accumulated runtime can become 20 + 15 = 35 minutes. The timer does not return to zero simply because its enabling rung becomes FALSE.
Real example – motor run-hour tracking (oil & gas plant)
A pump needs bearing service every 2000 hours. The RTO accumulates only while the pump is running. At the end of each shift the operator can leave the value alone. When total hours reach the preset, a maintenance bit turns on.
Ladder idea:
Pump running feedback enables the RTO.
A separate reset push-button drives the RES instruction.
Other common uses
- Total batch mix time across pauses
- Filter service life
- Production time excluding planned stops
- Pump runtime
- Motor runtime
- Filter operating hours
- Maintenance intervals
- Equipment usage
How do you reset an RTO?
On Allen-Bradley Logix systems, an RES instruction is commonly used to reset an RTO. The reset condition should be deliberate. For example, maintenance personnel may reset a runtime counter after scheduled service.
Do not assume that retaining the timer value when the rung goes FALSE means it will always survive a PLC power cycle. Power-loss behavior depends on the controller, memory configuration, and implementation.
Side-by-Side Comparison
| Timer | What it delays | Resets when | Needs RES? | Best for |
| TON | Turn-on | Input false | No | Start delays, debounce, alarms |
| TOF | Turn-off | After delay | No | Fan run-on, clear times |
| RTO | Accumulated time | Only with RES | Yes | Run hours, total process time |
The easiest way to remember them:
TON delays ON.
TOF delays OFF.
RTO remembers accumulated time.
How to Choose the Right Timer
- Need the output to wait before turning on? Use TON.
- Need the output to stay on after the input turns off? Use TOF.
- Need total time across starts and stops? Use RTO and add a reset rung.
Use TON when an action needs an ON delay.
Use TOF when an output needs to stay active for a period after its input turns OFF.
Use RTO when operating time must accumulate across separate running periods.
For example:
Need a motor to start 5 seconds later? TON
Need a fan to run 60 seconds after shutdown? TOF
Need to track total pump runtime? RTO
The process requirement should determine the timer, not the other way around.
Draw the timing on paper first. That single step prevents most mistakes.
Practical Ladder Patterns You Can Use Today
Motor start warning (TON)
Start PB + stop NC → TON (5 s)
TON.DN → motor run output
TON.TT → warning horn
Fan run-on (TOF)
Motor run → TOF (3 min)
TOF.DN → cooling fan
Runtime maintenance (RTO)
Motor running → RTO
RTO.DN → maintenance alarm
Reset PB → RES of the RTO
These three patterns cover most of the timer work on a typical plant.
Common Mistakes That Stop Machines
- Using TON when the machine needs a run-on delay. The fan stops too soon.
- Forgetting the RES on an RTO. The done bit stays on forever.
- Wrong time units. In Logix, PRE = 5 means 5 milliseconds, not 5 seconds. Use 5000 for five seconds.
- Putting the timer in a routine that is not scanned. The timer never updates.
- Using the same timer address for two different jobs. Behaviour becomes unpredictable.
TON keeps resetting
Check whether the enabling rung is becoming FALSE before the preset is reached.
RTO does not reset
Check the reset instruction and its enabling condition.
Timer reaches the wrong time
Check the preset, time base, PLC instruction, task execution, and platform documentation.
TOF does not behave as expected
Check the exact TOF instruction and how its output or status is defined on your PLC.
Timer does not appear to count
Check the enabling condition first. Then check whether the timer instruction is actually being executed and whether the PLC is in the expected operating mode.
When a timer fails on site, check these points in order:
- Is the rung true?
- Is EN on?
- Is ACC increasing?
- Is the preset correct?
- Is the routine being scanned?
How to Test a PLC Timer
Do not test only at the preset value.
If the preset is five seconds, test the behavior:
- Just before five seconds
- At five seconds
- Just after five seconds
- With the enabling condition removed early
- After enabling it again
- After a reset
Watch the timer’s accumulated value and status bits while testing. This can reveal a logic problem that is not visible during normal operation.
Program the timer, then force the input on and off while watching ACC and the status bits. Confirm the done bit turns on and off exactly when you expect. Do this on the programming laptop before the plant start-up. That five-minute test saves hours of downtime later.
Platform Notes
Allen-Bradley Studio 5000
- TON
- TOF
- RTO
- RES instruction to reset
- PRE (preset)
- ACC (accumulated value)
- EN (enable)
- TT (timing)
- DN (done)
- Presets are in milliseconds
Siemens TIA Portal and IEC-style timers
- TON
- TOF
- TONR (retentive timer)
- IN
- PT (preset time)
- ET (elapsed time)
- Q (output)
- Time is entered as T#5S
- Reset is done with the R input on TONR
The basic timer ideas are similar, but the programming details differ. Do not copy an Allen-Bradley timer example directly into Siemens TIA Portal and expect identical syntax or status behavior. Always check the instruction available for the PLC and programming environment being used.
The logic is the same. Only the names and time format change.
What Is a TP Timer?
A TP timer is a pulse timer available on many PLC platforms.
Instead of using a timer mainly to delay an ON or OFF transition, TP can generate an output pulse for a defined period.
A typical use is a solenoid or actuator that must remain active for a fixed time.
TP is useful, but TON, TOF, and RTO cover the main timer behaviors discussed here.
Final Field Advice
Use clear tag names such as T_MotorStartDelay or T_FanRunOn. Future technicians will thank you.
This is the practical knowledge you need on the job today. Apply the patterns above on your next motor, fan or runtime application and the logic will behave as expected.
Frequently Asked Questions
What is a TON timer?
TON is a Timer On-Delay. It starts timing when its enabling condition becomes TRUE and reaches its done state after the preset time. The output waits before turning on.
What is a TOF timer?
TOF is a Timer Off-Delay. It starts its delay when its input condition becomes FALSE. The output stays on for a set time after the input drops.
What is an RTO timer?
RTO is a retentive timer that keeps accumulated time when its enabling rung becomes FALSE. You need a reset instruction to clear it.
What is the difference between TON and TOF?
TON delays an ON action. TOF delays an OFF action. Use TON when you need a wait before something starts. Use TOF when you need something to continue running after the trigger stops.
What is the difference between TON and RTO?
A normal TON resets its accumulated time when its enabling condition becomes FALSE. An RTO keeps the accumulated value until you reset it with a separate instruction.
What do PRE and ACC mean?
PRE is the target or preset time. ACC is the time accumulated by the timer so far.
How do you reset an RTO?
The exact method depends on the PLC. Allen-Bradley Logix systems use an RES instruction. Siemens uses the R input on a TONR block.
Are PLC timers the same on Siemens and Allen-Bradley?
The basic timing concepts are similar, but instruction syntax, parameters, status information, and behavior can differ. Always check your platform documentation.
Why does my PLC timer keep resetting?
The first thing to check is whether the timer’s enabling logic is becoming FALSE before the preset time finishes.
Can an RTO survive a PLC power cycle?
Do not assume it will. Power-cycle retention depends on the PLC platform and memory configuration. Check your controller manual.
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External Reference: Rockwell Automation Official Documentation
