What Are Timers In PLC Programming


Timers are an essential part of PLC programming and are an important part of ladder logic. If you’re interested in becoming a PLC technician, understanding how they work and how they’re used is imperative. Timers in PLC ladder logic are basically just instructions. They provide preset demands that control the actions of the computer.

In this article we’ll explore the different types of timers found in PLC programming and their basic functions. You’ll also learn about the different training programs we offer, and how you can get started with PLCs. .

What Are PLC Timers?

Timers are devices that measure specific intervals of time and can be found in almost every facet of our lives. We use them when we cook our food, play sports, and even while we drive. But when it comes to PLCs, timers appear a bit differently. Unlike a kitchen timer or stopwatch, you cannot hold a PLC timer in your hand. A PLC timer is implemented into software that is in turn built into the controller. The timers used in PLCs are called electronic timers. They’re typically made from quartz clocks and are highly precise.

But why do we need timers in PLCs in the first place? Timers are an important part of PLCs because they delay actions. In many instances, a PLC timer will control a motor and that motor will only need to be running for a certain amount of time. In order for this to work properly, without the use of manual labour, a PLC timer will facilitate the task. Each PLC timer has an internal clock, an accumulator, and a counter value register. A technician will create instructions for the specific time delays needed. A PLC can have more than one timer, but that amount is dictated by the CPU memory.

What Are the Different Types of PLC Timers?

There are three main types of PLC timers: on-delay timers (TON), off-delay timers (TOF), and the retentive timer (RTO).

On-Delay Timer

On-delay timers are the most used timers in PLC programming. Basically, an action will not occur in a machine until the preset time has been reached. They delay the contact changeover until the timer has received an “on” signal. We can use a conveyor belt in a factory as an example of where an on-delay timer might be used. A technician could program the timer so that in order for the conveyor belt to turn on, the “start” button would need to be held down for a specific amount of time.

Off-Delay Timer

An off-delay timer is similar to its on-delay counterpart, as the name suggests, has the opposite function. With this timer, the output changes to “off” after a predetermined delay. So if you cut the power supply on a machine that has an “off” delay programmed into it, the connection to the circuit will still exist for as long as the preset timer tells it to. Using our conveyor built example again, a PLC technician can program a timer so that even after an “off switch” is pressed, the conveyor built will delay this action for a preset amount of time.

Retentive Timer

RTOs are similar to on-delay timers and they’re set up the same way. The difference is that retentive timers are able to “retain” or remember their status, so if a circuit is shut off and turned back on again, the time will start from where it left off instead of restarting. Their accumulated value acts differently when the selector switch is either turned on or off. When turned on, the accumulated value starts counting time and storing it. This means that if there was a change in the rung state or the system lost power, the timer would not be impacted.

PLC Timer Examples

PLC timers are used across industries for time delays and production monitoring. An everyday example of this type of timer would be a traffic light. In a four-way stop, a traffic light would have a programmed timer inside its mechanism, controlling when the lights change, depending on preset rules.

Within the manufacturing industry, PLC timers are responsible for keeping everything functioning the way that it should and providing added safety. Preset time values are added to machines so that they don’t turn on accidentally. Off-delay timers can be used for cooling fans or pumps, so that if all the other machines around it turn off, the fans will stay on. As mentioned before, you can have more than one timer programmed at the same time for separate tasks. Let’s say we want to control a pump. One pump can start after a 5 second delay. The second one can start immediately, and then turn off after 5 seconds. The third one can begin with one pulse, and then turn off after a 10 second delay.

By using a programming language, such as ladder logic, you can narrow down the exact units you want to use when setting a PLC timer—even down to the millisecond. To learn ladder logic and to be able to understand I/O timer commands, it’s best to take a PLC training course where you can gain hands-on experience.

PLC Technician Courses to Kickstart Your Career

At George Brown College, our PLC Technician Training Programs give students a thorough overview of PLC timers and hands-on experience writing ladder logic programs.

If you’re interested in becoming a PLC Technician, a big step forward is learning about the function of timers and how to program them. You can find out more about the PLC program we offer and its module on timers by reviewing the program outline.