Why is RTO used
in the place of T-ON?
are one of the most used instructions in a PLC program.
get started with the topic we need to know why the timers are used in the first
are used to keep a set of outputs ON/OFF for the specified amount of time.
example, I had to run a heater for 15 seconds I would go for a timer to make it
there are two types of an ON delay timer
in RSlogix 500,
1) T-ON - TImer On delay timer.
2) RTO - Retentive TImer.
consider an application we are programming requires a heater to be turned on
for 15 seconds after a level sensor has sensed the level is high enough.
we’re gonna write the logic for that using a TON.
we completed our logic,
● I:0.0/0 - It is the sensor
● o:0.0/0- It is the output for
● T4:0/TT - It is called the
timer timing bit.
following are the specifics on Timer block,
● Timer - The timer address has
to be specified here.
● Time Base - The time base is
the one that determines how the timer runs whether for milliseconds or
microseconds or seconds. It basically multiplies with the preset value. I have
mentioned here 1.0
● Preset - Here is where one
should specify the time the timer has to be active. So (1.0*15 = 15 seconds)
● Accumulator - Here is where
you can see the value of the seconds the timer has run so far.
we go to run time you can see that our process is working fine.
logic is working fine until when there is an emergency shut down or power shut
down in a plant. Imagine that the PLC had to turn OFF.
had to turn ON again after few minutes. Now the process had to repeat where it
left of meaning from 4th second the timer has to start running. But it won’t work
like that once there is a shut down in a PLC system the timer had to reset on
You can see the above figure for understanding.
to overcome this situation we’re gonna use an RTO timer to develop the logic.
is no difference in creating the instruction between TON and RTO except we have
to use a Reset here to reset the timer.
now everything is working fine and the PLC system goes off.
can see in the above image that the accumulator value is getting stored even
after the PLC goes OFF.
now when the system is turned ON again the program will be executed from where
it left off the timer will run from 5th second as you can see below.
when the sensor goes OFF the timer will store the accumulator value till it
the timer is being reset the accumulator value stays. Once it is reset the
accumulator value becomes zero.
that the input to reset should always act like a push button or One-shot (ONS)
and both the input to the timer and the input to the Reset shouldn’t be On at
the same time.
is the reason why RTO is used mostly in real-time applications.