The IEC Timer Standard for S7-1200 PLCs

In the world of industrial automation, time is the fourth dimension of logic. A Programmable Logic Controller (PLC) does not just react to "what" is happening (inputs), but also to when it happens and for how long. The Siemens S7-1200 PLC utilizes the IEC 61131-3 standard for timers, providing a robust, predictable, and highly accurate method for controlling temporal events.

This manual explores the four fundamental IEC timers TON (On-Delay), TOF (Off-Delay), TP (Pulse), and TONR (Accumulator) through the lens of a practical four-motor control system.

2. Theoretical Foundation: How Timers Work in S7-1200

Unlike old legacy timers that relied on limited hardware registers, IEC timers in the S7-1200 are "Function Blocks" (FBs). When you call an IEC timer in TIA Portal, the system creates a Data Block (DB) to store the timer’s status, including its start time, elapsed time, and current state.

IEC Timer Block

• IN (Input): The digital signal that triggers the timer.
• PT (Preset Time): The target duration. In TIA Portal, this is written in the format T#10s, T#5m20s, or T#100ms.
• Q (Output): The resulting logic state that controls your hardware (e.g., a Motor).
• ET (Elapsed Time): A dynamic value showing exactly how much time has passed since the timer started.
• R (Reset): (Specific to TONR) The signal used to clear the stored time value.

The Four Essential Timers

I. TON: The On-Delay Timer ("The Delay Starter")

The TON timer is the most common instruction in automation. Its primary purpose is to wait for a specific condition to remain "True" for a set period before acting.

 

Industrial Application: Imagine a conveyor belt. When a sensor detects a box (Input), we don't want to trigger a reject arm immediately. We wait 2 seconds (TON) to ensure the box is perfectly cantered.

Logic Behaviour:

1. When IN transitions from 0 to 1, the timer starts counting.
2. If IN stays at 1 until ET reaches PT, the output Q turns ON.
3. If IN drops to 0 at any point, ET immediately resets to 0, and Q turns OFF.

 

The Off-Delay Timer

The TOF timer is the inverse of the TON. It is used to keep an output active for a period after the input signal has disappeared.

Industrial Application: Consider a cooling fan for a high-temperature oven. When the oven is turned OFF (Input goes to 0), we want the fan (Output) to continue running for 5 minutes (TOF) to prevent heat damage.

Logic Behaviour:

1. When IN is 1, Q is immediately 1.
2. When IN transitions from 1 to 0, the timer starts counting.
3. Q remains 1 until ET reaches PT.
4. Once the time expires, Q turns OFF.

The Pulse Timer

The TP timer generates an output for a fixed duration, regardless of how long the input signal lasts. It is a "Fire and Forget" instruction.

Industrial Application: Think of an automatic lubrication system. When a machine starts (Input pulse), a pump (Output) needs to run for exactly 10 seconds (TP) to oil the gears. It doesn't matter if the machine start signal was a short tap or a long hold; the oiling time is fixed.

Logic Behaviour:

1. A 0 to 1 transition at IN triggers the timer.
2. Q turns ON immediately.
3. Q stays ON for exactly the duration of PT.
4. Even if IN changes back to 0 or pulses again during the count, the timer ignores it until the pulse is finished.

TONR: The Time Accumulator

The TONR is a specialized version of the TON. While a standard TON resets if the input is lost, the TONR retains the elapsed time.

Industrial Application: Maintenance tracking. A motor needs servicing after 100 hours of total runtime. The TONR accumulates time only when the motor is running. If the motor stops for a break, the timer pauses. When the motor restarts, it continues from the last second.

Logic Behaviour:

1. When IN is 1, the timer counts.
2. When IN is 0, the timer pauses but keeps the ET value.
3. When IN is 1 again, it continues counting from the paused value.
4. Q turns ON once the accumulated time reaches PT.
5. The timer only resets to 0 when the R (Reset) input is activated.

Problem Solution: The Four-Motor Control System

To demonstrate these timers in a real-world scenario, we will implement a program for a factory floor with four distinct motor behaviours.

 

Hardware Address Mapping

Component	PLC Address	Timer Type	Goal
Switch 1	I0.0	TON	Start Motor 1 after a 10s delay.
Switch 2	I0.1	TOF	Stop Motor 2 10s after the switch is turned OFF.
Switch 3	I0.2	TP	Run Motor 3 for exactly 10s upon a tap.
Switch 4	I0.3	TONR	Accumulate 10s of runtime for Motor 4.
Reset Button	I0.4	—	Reset the Accumulator (Motor 4).
Motor 1	Q0.0	—	Output 1
Motor 2	Q0.1	—	Output 2
Motor 3	Q0.2	—	Output 3
Motor 4	Q0.3	—	Output 4

 

Network Logic Description

Network 1: Safety Delay (Motor 1)

In this network, we use the TON instruction. When the operator flips Switch 1 (I0.0), a siren might sound, but the motor does not move. After exactly 10 seconds, the TON instruction evaluates that the condition has been met and sets Q0.0 to High. This ensures that the motor only starts if the switch is held or locked in the ON position.

Network 2: Cool-Down Logic (Motor 2)

Here, the TOF instruction is used. As soon as Switch 2 (I0.1) is turned ON, Motor 2 (Q0.1) starts spinning. When the operator finishes their work and turns the switch OFF, the TOF starts its 10-second countdown. This keeps the motor running (perhaps for ventilation) before finally shutting down automatically.

Network 3: Precision Pulse (Motor 3)

In this network, we use the TP instruction. This is perfect for a "Start" push-button. When Switch 3 (I0.2) is pressed, Motor 3 (Q0.2) triggers. Even if the operator releases the button or gets distracted and presses it again, the motor will run for exactly 10 seconds—no more, no less.

Network 4: Runtime Accumulation (Motor 4)

Using the TONR, we track the total time Switch 4 (I0.3) has been active. If the operator runs the motor for 4 seconds, then stops for lunch, and then runs it for 6 more seconds, the timer hits the 10-second mark. Motor 4 (Q0.3) then activates. This motor will stay ON forever until the Reset (I0.4) signal is received, clearing the memory of the timer.

Runtime Test Cases & Validation

To ensure the program is working correctly in TIA Portal, engineers must perform "Runtime Testing." Below are the expected results:

Test Sequence	Input State	Timer State	Output State	Result
1	I0.0 = 1	TON counting	Q0.0 = 0	Motor 1 waiting...
2	I0.0 = 1 (after 10s)	TON Done	Q0.0 = 1	Motor 1 Starts
3	I0.1 = 0 (was 1)	TOF counting	Q0.1 = 1	Motor 2 still running...
4	I0.1 = 0 (after 10s)	TOF Done	Q0.1 = 0	Motor 2 Stops
5	I0.2 = 1 (pulse)	TP counting	Q0.2 = 1	Motor 3 runs for 10s
6	I0.3 = 1 (intermittent)	TONR adding	Q0.3 = 0	Motor 4 accumulating...
7	I0.4 = 1	TONR Reset	Q0.3 = 0	Time cleared to 0s

 

Best Practices for TIA Portal Timers

Data Block Management: Every IEC timer requires a DB. In large programs, use "multi-instance" DBs to keep your project tree organized rather than having hundreds of individual Timer DB

Scan Cycle Awareness: PLCs execute code from top to bottom. If a timer finishes in the middle of a scan, the outputs further down the program will react immediately, while outputs above it will wait for the next scan.

Memory Retention: If the PLC loses power, standard timers usually reset. If you need a timer to remember its value after a power failure, ensure the Timer DB is marked as "Retentive.

Avoid Using 'Global' Timers for Local Logic: Always try to keep your timers local to the Function Block they are serving to prevent "crosstalk" or logic errors where two different parts of the program try to use the same timer.

 

The implementation of IEC timers in the S7-1200 PLC is a foundational skill for any automation engineer. By mastering the differences between ON-delay, OFF-delay, Pulse, and Accumulator logic, you can design systems that are not only functional but also safe and energy-efficient. Whether it is delaying a motor start for safety or ensuring a cooling fan runs after a stop, timers provide the precision required for modern industrial excellence.