PLC ladder logic timers are often used in PLC programs in many different ways.
In this article, I will discuss types of PLC timers. I will explain what they are, how they work, the different types of timers, and give some examples of how they are used.
PLC timers are internal PLC instructions that can be used to delay input and output signals in the PLC program.
These timers operate like relay timers but you cannot hold a PLC timer in your hand and they do not need to be connected to wires to operate.
Every PLC vendor incorporates timers in their programming languages, and their operation is essentially the same across programming platforms. We will be referring to Rockwell timers used in Allen Bradley PLCs.
Types of PLC timers
There are three main types of timers in PLC programs:
- The on-delay timer,
- The off-delay timer,
- The retentive on-delay timer.
PLC timer values
All of these PLC timers have a file name, a preset value, an accumulated value, a timer enabled bit, a timer timing bit, and a timer done bit.
Timer file name
The timer file name is referred to as a PLC tag. Tags can be found in the Controller Tag or Program Tag List. Drilling down into the timer file name tag reveals the timer’s preset and accumulated values, as well as the timer's enabled, timing, and done bits.
Some PLC programming languages create the timer file name for you, and others allow you to create it.
For all of my timer examples, I will use the Timer1 file name. I will also use a normally open instruction called selector switch to enable and disable the timers.
Timer preset value
The timer preset value shows the timer's delay length. In our case, the unit of time is milliseconds.
You can change this number by typing it in yourself or having the PLC write a number into it from another source. This preset value does not change during the timer’s normal operation.
Timer accumulated value
The timer's accumulated value is the number that shows how long the timer has been timing.
The timer accumulated value does not function the same on all of these timers. I will explain how this value accumulates time and how each timer works.
Timer enabled bit
The timer enabled bit will be on when the timer is enabled. In our case, whenever our Selector Switch input is TRUE.
Timer timing bit
The timer timing bit will be on while the timer is timing.
Timer done bit
The timer done bit does not function the same on all of these timers. I will explain how the timer done bits works as I explain how each timer works.
PLC timer examples
For my PLC timer examples, I will be using Studio 5000, which is the PLC programming software for Allen-Bradley ControlLogix PLCs.
But, as we said earlier, what you will learn here in these examples can be applied to other PLC programming software.
On-delay timer
For the first example, I will talk about the on-delay timer.
Let’s start by placing a new on-delay timer instruction on a PLC rung with a selector switch input.
Type the timer file name Timer1.
Next, enter the timer preset so that it equals ten seconds. Notice that I typed ten thousand, I did this because the time base is in milliseconds and ten thousand milliseconds equals ten seconds.
Not all PLC timers use the same time base, some have time bases that cannot be changed while others will give you multiple time base options.
This PLC on-delay timer is now set up and ready to use.
How does an on-delay timer work?
Next, I will discuss what happens to the PLC on-delay timer when the selector switch gets turned on and off.
- When you turn on the selector switch in the PLC timer diagram, the timer enabled bit and timer timing bit turn on, and the timer’s accumulator starts accumulating time.
- If you leave the selector switch on, the timer’s accumulated value will reach ten thousand. When it does, the timer will stop accumulating time, the timer timing bit will turn off, and the timer done bit will turn on.
- At this point, when you turn off the selector switch the timer accumulated value will reset back to zero, and the timer enabled and timer done bits will turn off.
- If you turn off the selector switch anytime before the timer is done timing, the timer accumulated value will reset back to zero, and all of the timer bits will be off.
On-delay timer applications
On-delay timers are used a lot in PLC programs and in many different ways. For example, an on-delay timer can be used on a conveyor’s start button.
The timer could be set up so that the start button would need to be pushed and held for five seconds before the conveyor would start.
Another on-delay timer could also be set up to sound a conveyor start-up warning horn and light.
Off-delay timer
Now let’s talk about off-delay timers.
Setting up an off-delay timer is the same as setting up an on-delay timer. Place the timer off-delay instruction on a PLCprogram rung with a selector switch input and set ten thousand as the timer’s preset value.
How does an off-delay timer work?
Next, I will discuss what happens to the PLC off-delay timer when the selector switch gets turned on and off.
- When you turn on the selector switch, the timer’s accumulated value resets to zero, and the timer enabled bit and the timer “done bit” turn on.
- When you turn off the selector switch, the timer accumulated value will start accumulating time, the timer enabled bit will turn off, the timer timing bit will turn on and the timer done bit will stay on.
- If you leave the selector switch off, the timer’s accumulated value will reach ten thousand. When it does, the timer will stop accumulating, the timer timing bit and the timer done bit will turn off.
- If you turn on the selector switch in the plc timer diagram before the timer is done timing, the timer accumulated value will reset back to zero, the timer enabled bit will turn on, the timer timing bit will turn off, and the timer done bit will stay on.
Off-delay timer applications
Off-delay timers are used when you need something to stay running longer than everything else. For example, cooling fans and pumps might continue to run after a machine pauses from doing work.
Retentive on-delay timer
Retentive on-delay timers are a lot like on-delay timers. They get set up the same way and the bits react in the same way.
The only major difference with retentive on-delay timers is how the timer's accumulated value acts when you turn the selector switch on and off. You also need a reset bit.
How does an retentive on-delay work?
- For an retentive on-delay, when the selector switch is turned on, the retentive on-delay timer accumulated value starts accumulating time, the timer enabled bit turns on, and the timer timing bit turns on.
- If the selector switch is left on, the accumulated value will reach ten thousand, the timer will stop accumulating time, the timer enabled bit will stay on, the timer timing bit will turn off, and the timer done bit will turn on.
- At this point, if the selector switch is turned off, the accumulated value will stay ten thousand, the timer enabled bit will turn off, the timer timing bit will stay off, and the timer done bit will stay on.
- If the selector switch is turned back on, only the timer “enabled bit” will turn on, everything else will stay the same.
A timer reset bit will need to be added to the PLC program to reset the accumulated value of a retentive on-delay timer. To trigger this reset bit, I will use a momentary push button to operate a normally open contact called Reset On.
- When the selector switch is off and the timer reset button is pushed, the timer accumulated value will reset to zero and the timer done bit will turn off.
Retentive on-delay Applications
Retentive on-delay timers can be used to track the runtime of equipment. They can be set up to trigger an indicator light to let someone know the equipment has run long enough and requires maintenance to inspect and clean it.
Summary
In review, this article has taught you about the three main types of timers in a PLC:
- The three main types of PLC timers:
- On-delay,
- Off-delay,
- Retentive on-delay.
- You also learned that these timers have:
- A preset value,
- An accumulated value,
- An enabled bit,
- A timing bit,
- A done bit.
- You have also learned some examples of where these timers are used. For example, using timers to sequence the operation of a machine is called time-based sequencing.
Got a friend, client, or colleague who could use some of this information? Please share this article.
