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PLC Timer Types | On-Off Delay Timer Programming Examples

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Everybody knows why time is so important in our life. Whether it is human or machine, nothing can be done without time.

Every little thing around us is getting automated. Here we are interested in PLC (Programmable Logic Controller) automation.

In PLC automation, different types of PLC programming instructions are used with theirs different specifications. Out of these PLC instructions, timer instruction is one of the most important instruction which plays a very significant role.

In this tutorial, I am describing the PLC timer in detail with the programming instructions and functions.

Let’s start from the beginning.

What is the PLC Timer?

A timer is one of the most essential and useful entity.

The role of PLC timer is to control and to operate the device for a specific duration. With the timer, we can perform any specific operations for a particular time span.

You can set the timer with simple PLC programming instructions. Every PLC having different timer functions.

Timers are used to provide programming logic and to decide when to turn on or off the circuit.  It has both normally open (NO) or normally closed (NC) contact.

Representation of the I/O Timer Contact:

Timer Output contact is shown in the coil form or box form or rectangular form.

In AB and Siemens PLC, it is represented in the box shape.

If you want to perform work or device activity in a particular time span, you have to get familiar with the timers. For this, you have to learn I/O timer instructions for writing the PLC program.

In the Ladder Diagram (LD) PLC programming, you can set the PLC timer from millisecond (ms) to an hour (hr) time range.

Are you interested in knowing, how does it actually work?

Let’s see the internal circuit of the timer.

Basic Internal Circuit of  Timer

Now, we are looking at the internal timer circuit of the PLC. The working of the timer circuit is based on the four main parts. These are as follows.

  • Input Signal
  • Internal Power Supply
  • Digital Timer Display
  • Output Signal

This is how they are connected and constructed in the given diagram.

Internal Structure Of the PLC Timer

Each of the Internal parts of the timer circuit has various features and functions.

Here are some of the basic terminologies you need to understand about Timer used in the PLC.

Timer Auxiliary Power Supply (APS):

The input power supply is provided for the proper functioning of the timer circuit. This can be connected with the AC or DC supply like 230V AC or 5/10 V DC.

Timer Start or Set Operation Signal:

If the auxiliary power supply is ‘on’, the timer will give the momentary input pulse for the given circuit.

Reset Timer Signal:

The device or other systems can be reset by switching the APS in the ‘on’ or ‘off’ condition.

Output Function:

There are multiple output functions.  It helps to select the proper functions for the applications.

The output gets activated as an output signal of the timer circuit.

Timer Display:

The digital timer displays the set and elapsed timing value. For the automation purpose, the values can be displayed in a few milliseconds (ms). This will be easy for tracking your automation system.

Types of the PLC Timer

What are the different types of timers instruction used in PLC programming?

The classification of the PLC programming timer is-

  1. Timer On Timer (TON)
  2. Timer Off Timer (TOFF)
  3. Retentive Timer (RTO)

Let’s see them briefly one-by-one.

1. Timer On Timer (TON)

TON is called On delay timer. It helps to start momentary pulses and activated the output contact based on the delay time.

In the Programming, when an On-delay timer is energized (True), it delays turning ‘on’ the timer’s output. This output will be ‘on’ until the timer’s preset time value is reached.

A simple construction of the AB PLC On-delay timer

Example: Running Electric Motor after 10 seconds.

– If you press the button (NC), it starts the momentary pulse. After the 10 seconds, the motor will be ‘On’.

2. Timer Off Timer (TOFF)

It is also known as an Off-delay timer. It helps to switch off the output or system after a certain amount of time.

In PLC programming, when the off-delay timer is energized (True), it immediately turns ‘on’ its output. The output will be ‘on’ till it reaches the setting time.

When it reaches preset time, the output turns ‘off’. Due to the turning ‘off’ condition, the timer is de-energized (False).

A simple construction of the AB PLC Off-delay timer

Example: Stop electric motor after the 10 seconds.

– Firstly, you should switch press (NC), the motor will ‘on’ for the 10 seconds. After 10 seconds, the motor will automatically stop (NO).

This is the main function of the off-delay timer.

3. Retentive Timer On (RTO)

The main function of the RTO is used to hold or store the set (accumulated) time.

It is used in the case when there is a change in the rung state, power loss or any interruption in the system.

In the AB PLC, retentive timer instruction look like this.

These are the three general types of PLC timers.

 Timer Instructions Address for Multiple PLC Brands

We have seen three timers provide the time delay functions to control the PLC operations. There are four main values that timer deals with.

  • Timer Address
  • Preset Value
  • Timer Base Value
  • Accumulated value

Each timer instruction has three very useful status bits. These bits are…

  • Enable bit (EN)
  • Timer Timing bit (TT)
  • Done Bit (DN).

In the AB and Siemens PLC, the output bit is often called the timer’s ‘Done bit’. And it indicates the timer has reached its preset time.

1. Addressing for ABB PLC

In the ABB PLC, we can simply write the I/O address of the ladder diagram. We can set the value in ranges from T0 to T255.

You can see the above diagram of the I/O contact representation.

2. Addressing for AB (Rockwell) PLC

For the AB PLC, the timer has the address ranging from T4:0 up to T4:255.

Where T4 is the file type.

Addressing format for timer instruction with the three status bits.

File tpye: Element Number/ Bit status
  • Enable bit (EN) address is ranging from T4:0/EN up to T4:255/EN
  • The addressing for Timer timing bit (TT) is ranging from T4:0/TT up to T4:255/TT
  • Done bit (DN) address is ranging from T4:0/DN up to T4:255/DN

3. Addressing for Siemens PLC

In the Siemens, LD program can be written with the five types of timers.

  • Pulse timer (S_Pulse)
  • Pulse extended timer (S_PExT)
  • On delay timer (S_ODT)
  • On delay extended timer (S_ODTS)
  • Off delay timer (S_OffDT)

The general block diagram of the timer (In Siemens PLC),


S – Set value or signal for the timer

TV – Time Variable. It is used to store time value in the form of


You can enter the time value from 1 to 9.990 seconds.

R- Reset value of the timer

Q – Output of the timer

BI – Current time in binary code

BCD – Current time in binary decimal code

4. Addressing for Delta PLC

For the WPLSoft software, you can use timer addressing ranging from ‘T0 to T127’.

In Delta PLC, input timer address is shown like general representation (T0, T1,…….. T127).

And Output coil is written in the form of

T(Address rang) K(10*timer value)


‘T0’ is timer address and   ‘K’ is the constant term

Block Diagram Of Delta PLC timer

For Delta PLC, the timer starts for a 10-second.  It should be written in the form of ‘T0 K100’.

5. Addressing for Mitsubishi PLC

Both, Mitsubishi PLC and Delta PLC, use the same timer addressing format.

Example-based on the PLC Timer Instruction

The very basic and real-life example is controlling traffic signals using PLC automation.

After a certain (fixed) time,  each side signal has to turn on and off. At a time only one side traffic signal should be on.

This logic can be implemented using simple PLC timers.

Applications of Timer Instruction:

Here are some of the basics applications of timer you can use in a PLC automation environment.

  1. Use for the delay action
  2. It is used to run or stop operation as per the user’s command.
  3. The RTO timer helps to record or hold an intermediate time value.

This is all about PLC timer. This is a really big topic. I tried to make it simple. If you have any questions, feel free to ask in the comment.

If you are new to the PLC programming, I have shared my thought about how you can master and learn PLC for free.

Happy PLC Learning!

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