TOF timer help

bedazzled

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Hello there!
I have an assignment to use a TOF timer to delay the start of a motor ( this means that the TOF timer would work as a regular TON would ).
I've experimented with different bits (EN, DN, TT)and different contacts(NC, NO) but i never seem to get the required result.

I understand that TOF timers are usually used to delay the shutdown of a motor and i would be thankful if someone has an idea how to solve this.

Thanks in advance :)
 
The Logic of an 'OFF DELAY TIMER' (TOF) is to commence timing and change state after power has been taken away from the timer

so in a motor control circuit you would turn the power off to the tier and let it stop the motor by its timed N/C contact after the time period.
( there are other ways)

welcome to the site
 
this is a "school" or training assignment – right? ...

if so, then there are several lessons to be learned here ...

TIP: one quick hint that helps many beginners when working with a TOF (Timer Off Delay) instruction is to substitute the word "Run" for the "Done" bit ...

at least that would help when the TOF is used correctly – for its intended use to "delay before turning OFF" ... sounds like your assignment is to use the TOF "backwards" – and make it delay before it turns something ON ...

regardless ...

here's one place to learn the lessons you're looking for – click the link in my signature (below) ... then click the menu for the "Sample Lessons & Videos" page ... scroll down to the link for the "PLC Email Quiz" page ... and download the Questions and the Answers (PDFs) for the "Beginner Level #121 Quiz - Surprises at Go-To-Run" ... the examples associated with Figure A should be very helpful to you ...

experiment with the ideas shown – you should be able to figure it out from there ...

IMPORTANT !!! one of the main/big/important (as in CRITICAL/HUGE/MONUMENTAL) ideas that your instructor should want you to learn from this assignment is the following:

it is possible to use "clever" but unorthodox programming arrangements to get a PLC to control the equipment under "normal" day-to-day operating conditions ... BUT ... these same arrangements may cause EXTREMELY DANGEROUS operation after a power failure – or in other unexpected situations ...
Click to expand...
we could be talking "lawyer level" ramifications here ...

good luck with your assignment ...
 
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Thank you guys for all the welcome and help.
This is actually a practice example for an upcoming quiz (test, exam or w/e)...
I'll definitely look over your examples and links so i hope i can figure something out :)
Thanks again!
 
Just remember to try different things NOW in your training to see how they work. No harm no foul and then you will have a better understanding of how things are going to react when it's online in a system where experimentation and trial and error is not welcomed. Good luck and welcome to the forum.


Kraken Fan #69
 
in Post #4, Lancie1 has given a good example of a potential "Surprise at Go-To-Run" effect that I mentioned earlier ...

the screen shot below is taken from an actual Allen-Bradley SLC-5/05 processor ...

notice that the TOF construction is capable of giving DIFFERENT results from the TON construction whenever the processor has a "Go-To-Run" event – even though – BOTH constructions would give identical results in normal day-to-day operation ...

these are the types of things that programmers usually learn about through bitter experience ...

scenario: suppose that a new piece of machinery is tested out – and it passes the Factory Acceptance test with flying colors ... specifically, it meets all of the programming specifications ...

but ...

eventually the machinery has a "Go-To-Run" event (for example: suppose that power is restored after a power failure) ... if the programmer hasn't taken the proper steps to accommodate these types of situations, damage to the equipment – and danger to personnel can easily result ...

NOTE: in the types of situations being discussed here, the LogixPro simulator that Lancie1 used for his example is capable of giving different results from the actual processor that I've used ... this is due to the fact that LogixPro will reset the TON's Accumulator at Go-To-Run (as expected) – but it does NOT reset the TON's Done bit ...

on the other hand, with a real Allen-Bradley processor, the PRESCAN operation would reset the TON's Done bit to a status of ZERO ...

.


ton_vs_tof.PNG
 
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Timer Off Delay

In addition to what the Folks have mentioned,

A timer of delay could be constructed from a timer on delay by inverting both the timer input & timer output.

Hagos
 
Lab vs Real Life application

bedazzled said:
Hello there!
I have an assignment to use a TOF timer to delay the start of a motor ( this means that the TOF timer would work as a regular TON would ).
I've experimented with different bits (EN, DN, TT)and different contacts(NC, NO) but i never seem to get the required result.

I understand that TOF timers are usually used to delay the shutdown of a motor and i would be thankful if someone has an idea how to solve this.

Thanks in advance :)
Click to expand...

I have programmed 1,000+ projects over 20+ years, never needed to use a TOF.

We have also inherited 100+ projects, programmed by others, we may have come across one or two TOF instructions, they were found in PLC programs that also had other quirky "holes" that our customers had to tolerate.

$0.02
 
hagos said:
In addition to what the Folks have mentioned,

A timer of delay could be constructed from a timer on delay by inverting both the timer input & timer output.

Hagos
Click to expand...

"could be", but i bet it would not behave in the same fashion during a "go-to-run" situation.

it is important to understand what the pre-scan does with TON and TOF, and understanding that will allow your logic to behave as intended in all situations.

it is so easy to forget the "pre-start", "power-up", or "mode-change" situations when designing logic using timers.

I worked with one client who's specification stipulated that only RTO (retentive) timers should be used... he'd obviously had fingers burnt in his past. No amount of attempted education would shift him from his viewpoint, and the specification remained unaltered.....
 
I have programmed 1,000+ projects over 20+ years, never needed to use a TOF.
Click to expand...
Exactly. They are just a curiosity to confuse and amaze the poor students. Enough time has been wasted on TOFs to have rebuilt the World Trade Center.
 
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now let's don't confuse the people who want to do it "right" ...

well, I don't want to start an argument – but actually the TOF instruction DOES have a perfectly valid use ...

several years ago, I was teaching an onsite contract training course for a company – and made the same demonstration that I mentioned in my post #3 above ...

I showed the class how both the TON and the TOF controlled outputs worked identically in normal day-to-day operation ... then I had a student switch the processor to Program mode – and then back to Run mode again ...

as soon as the output controlled by the "Backwards TON" started unexpectedly running, about three of the students all yelled (simultaneously – and in three-part harmony) "THAT'S what's doing it!" ...

turns out that they had a piece of machinery in their plant that SHOULDA-BEEN controlled by a TOF – but years ago, someone had programmed it using a "Backwards TON" instead ... everything ran perfectly in normal day-to-day operation ...

but ...

every time the power to the plant would go off – and then come back on again – or whenever the processor got switched into the Run mode – a certain little dispensing pump would immediately "chug-a-chug-a-chug" an unexpected dose of a certain chemical into a certain pipe ... they said it was a pain in the neck to dismantle the piping and clean the gunk out of the system – and so on ...

so ...

there are indeed a few valid instances where you SHOULD use a TOF ... they exist for a reason ... that's the only point I'm trying to make here ... feel free to ignore me ...
 
Last edited:
Agreed

Ron Beaufort said:
well, I don't want to start an argument – but actually the TOF instruction DOES have a perfectly valid use ...

several years ago, I was teaching an onsite contract training course for a company – and made the same demonstration that I mentioned in my post #3 above ...

I showed the class how both the TON and the TOF controlled outputs worked identically in normal day-to-day operation ... then I had a student switch the processor to Program mode – and then back to Run mode again ...

as soon as the output controlled by the "Backwards TON" started unexpectedly running, about three of the students all yelled (simultaneously – and in three-part harmony) "THAT'S what's doing it!" ...

turns out that they had a piece of machinery in their plant that SHOULDA-BEEN controlled by a TOF – but years ago, someone had programmed it using a "Backwards TON" instead ... everything ran perfectly in normal day-to-day operation ...

but ...

every time the power to the plant would go off – and then come back on again – or whenever the processor got switched into the Run mode – a certain little dispensing pump would immediately "chug-a-chug-a-chug" an unexpected dose of a certain chemical into a certain pipe ... they said it was a pain in the neck to dismantle the piping and clean the gunk out of the system – and so on ...

so ...

there are indeed a few valid instances where you SHOULD use a TOF ... they exist for a reason ... that's the only point I'm trying to make here ... feel free to ignore me ...
Click to expand...


Ron,
Thank you for opening my closed ears.
I instruct my peers to use EVERY PLC instruction in the palette.
Although I have an extensive background, I have only limited assembly and process exposure. I will study-up on the TOF.

Thanks
 

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