Types of PLC Timer | On-Off Delay Timer Programming Examples

In an earlier tutorial, we have studied the PLC (programmable logic controller) timer instruction with the help of the internal circuit diagram.

The multiple PLC brand software’s are having different timer instructions. Each brand of PLC software has specific timer functions for time-based programming.

Here, I am explaining types of PLC timer instructions which are used in AB PLC and Siemens PLC.

What are the types of the PLC Timer?

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

For ladder diagram programming languages, the three different types of PLC timer instruction are used.

These three timers are,

1. On Timer (TOn)
2. Off Timer (TOff)
3. Retentive On-Off Timer (RTO)

These PLC timer instructions consist of three words like ‘word 0’, ‘word 1’ and ‘word 2’.

Out of them, ‘word 0’ is consisting of three bits. These three bits very useful status bits in PLC programming instruction.

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

Let’s see PLC timer types briefly one-by-one.

1. On Timer (TON)

What is TON?

TON is called ‘On Delay Timer’.

An on-delay timer (TON) is a programming instruction which use to start momentary pulses for a set period of time.

Let’s see, a simple construction of the AB PLC On-delay timer programming instruction.

In the LD 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.off delay PLC timer instruction.

It helps to activate the output (like machine or process) contact based on the delay time.

Read More: You can read more about the Input and Output PLC Module.

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. Off Timer (TOFF)

What is TOFF?

TOFF is also known as an ‘Off-Delay Timer’.

A off-delay (TOF) timer is a PLC programming instruction which use to switch off the output or system after a certain amount of time.

See here, a basic structure of AB PLC Off delay timer programming instruction.

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).

It helps to delay the shut down of machinery or process in automation industries.

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/Off (RTO)

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

RTO 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.

This is all about the basic explanation of AB PLC timer instructions. These three type of PLC timer are used in different industrial applications.

In Siemens PLC, five different types of PLC timers are used. These five-timer instructions are on-delay, off-delay, pulse, pulse extended, and on-delay extended timer are used.

Want to write your own program based on the timer with the brand of PLC’s software?

You can easily write the Ladder Diagram programming by using the timer instructions address for multiple brands like AB PLC, ABB PLC, Siemens PLC, Mitsubishi PLC, and Delta PLC.

If you have any questions, feel free to ask in the comment.

Ready for Test:

If you are ready for online test, click on is a PLC Automation Quiz.

Thanks for Reading!

Dipali Chaudhari

I have completed master in Electrical Power System. I work and write technical tutorials on the PLC, MATLAB programming, and Electrical on DipsLab.com portal.

Sharing my knowledge on this blog makes me happy.  And sometimes I delve in Python programming.