In the world of industrial automation, PLC timer functions are among the most fundamental and widely used instructions. Timers allow programmable logic controllers to control time-dependent operations, from simple delay-on signals to complex sequencing in manufacturing processes. Whether you’re an automation engineer, a student, or a technician working with ladder logic, understanding how PLC timers operate is essential for designing reliable control systems. This comprehensive guide explores the various types of timer functions, their applications, programming techniques, and best practices.
What Are PLC Timer Functions?
A PLC timer function is an instruction that measures or controls the duration of an event within a programmable logic controller’s scan cycle. Timers count internal clock pulses (typically measured in milliseconds) and trigger outputs when a preset time value is reached. They are critical for operations such as:
- Delaying the start or stop of a motor
- Sequencing conveyor operations
- Generating pulse signals
- Enforcing safety lockouts
- Managing batching and dosing cycles
Every major PLC manufacturer—including Allen-Bradley (Rockwell), Siemens, Mitsubishi, and Schneider Electric—offers timer instructions, though naming conventions and behavior may vary slightly between platforms.
Common Types of PLC Timer Functions
There are several standard timer types used across PLC platforms. Each serves a specific purpose in control logic.
1. On-Delay Timer (TON)
The On-Delay Timer (TON) starts timing when its input condition becomes true. The output remains OFF until the accumulated time equals the preset value. Once the preset is reached, the output turns ON and stays ON as long as the input is energized. If the input turns OFF before timing completes, the accumulated value resets to zero.
2. Off-Delay Timer (TOF)
A TOF timer begins timing when the input transitions from true to false. The output remains ON during the timing period and only turns OFF after the preset time has elapsed. This is commonly used for cooling fans that run after a motor shuts down.
3. Retentive On-Delay Timer (TONR / RTO)
The retentive timer behaves like a TON, but it retains its accumulated value even when the input turns OFF. The accumulated time only resets when a separate reset instruction is executed. This is ideal for tracking total run time of equipment.
4. Pulse Timer (TP)
A pulse timer generates an output pulse of a fixed duration when triggered, regardless of how long the input remains true. The pulse starts on a rising edge of the input and ends after the preset time.
Comparison of Standard PLC Timer Types
The following table summarizes the behavior of the most common timer instructions found in modern PLCs:
| Timer Type | Input Trigger | Output Behavior | Reset on Input OFF | Typical Use Case |
|---|---|---|---|---|
| TON (On-Delay) | Rising edge (ON) | Turns ON after preset time | Yes | Start delay for motors |
| TOF (Off-Delay) | Falling edge (OFF) | Stays ON, then turns OFF | Yes | Cooling fan shutdown |
| TONR / RTO | Rising edge (ON) | Turns ON after preset time | No (requires RES) | Total run-time tracking |
| TP (Pulse) | Rising edge (ON) | Fixed pulse regardless of input | Yes | Flashing indicators |
How PLC Timer Functions Work Internally
Inside the PLC, timers rely on three primary components:
- Time Base: The resolution of the timer, such as 1 ms, 10 ms, 100 ms, or 1 s. The selected time base determines the smallest unit the timer can measure.
- Preset Value (PV): The target duration the timer must reach before activating its output.
- Accumulated Value (ACC): The current elapsed time stored in the timer’s memory word.
During each PLC scan cycle, the CPU checks the timer’s enable input. If true, it increments the accumulated value by the time base. When ACC ≥ PV, the timer’s done bit (DN) is set, signaling the output condition. The accuracy of a timer depends on the scan time—longer scan times can cause timing drift in fast applications.
Practical Applications of PLC Timers
PLC timer functions are used in virtually every industrial sector. Below are some of the most common real-world applications:
Conveyor Sequencing
In bottling plants, on-delay timers space out bottles on a conveyor. Each new bottle triggers a timer that delays the next operation until the bottle reaches the correct position.
Motor Star-Delta Starting
A classic motor control technique uses a TON timer to keep the motor in star configuration for a few seconds, then switches to delta. The timer ensures the transition happens only after the motor has reached sufficient speed.
Traffic Light Control
Timers alternate between red, yellow, and green phases. By chaining multiple TON or TOF timers, smooth and predictable traffic flow can be maintained.
Safety Interlocks
Timers enforce minimum lockout durations after emergency stops are pressed, preventing rapid restarts that could endanger personnel.
Best Practices for Using PLC Timers
To write robust and reliable timer logic, follow these proven guidelines:
- Choose the correct time base: Use 1 ms or 10 ms for high-speed applications, and 100 ms or 1 s for general-purpose timing.
- Use descriptive tag names: Instead of T4:1, name timers like Motor_Start_Delay to improve readability.
- Avoid cascaded timers when possible: Cascading introduces cumulative timing errors. Use a single timer with a calculated preset when feasible.
- Consider scan time effects: For sub-10 ms timing precision, use high-speed timers or hardware-based counters instead of standard PLC instructions.
- Reset timers explicitly: Always include a reset condition in your logic, especially for retentive timers like RTO/TONR.
⚠️ Warning: Never rely on PLC timers for life-safety functions such as emergency stop response or light curtain monitoring. Use certified safety relays or a dedicated safety PLC (e.g., Siemens F-CPU or Allen-Bradley GuardLogix) that meets ISO 13849 and IEC 62061 standards.
Troubleshooting Common Timer Issues
