OSF Instruction with SLC 50/4

Craig

Guest
C
I don't wish to start "my PLC is better than your PLC" debate.

Attempted to keep same programming principles of plant main PLC, (other than AB)to that of a new AB SLC 50/4.
Due to the Instruction Palette of RSLogixs, it couldn't be done,
the "OSF" instruction is not available for the SLC50/4.
I had to program it accordingly with equal success.

But does anyone know why the 50/4 Instruction palette doesn't
include the OSF, as other known brands of Medium PLCs have this
instruction?

And have found it to be a KISS instruction to use.

Thanks
 
OSF = ONE SHOT FALLING
OSR = ONE SHOT RISING

I understand the differences in the way they operate but what condition(s) would make one preferable over the other?

While we are at it maybe someone could put into words why either is used at all. I understand there are conditions that require that rungs only be triggered on one scan but can not put into words why. If someone could be kind enough to put this into words.

BTW Where is Allen & Terry these days? They are great at explaining these things.
 
Ron, when we are handling bit-space by words -16-32 bits debending or processors- it is easy produce both edges as shown in attached file.
File is for Simatic Step5. Please open it by NoteBook.
In my mind ONS is enough for helping application programming, but many times we are using edges for nothing.
 
I am a bit confused,

Can't the Input conditions can be reversed to get either OSR or OSF

ons.gif
 
Well, how about this:

You want to monitor that a pushbutton (Cycle Start) has just been pressed but has not necessarily released yet. You could use this information to drive a position monitoring register to zero. In this case you might use an OSR. The value here is that the cycle can immediately start, the OSR would reset the register value, and the machine could start its movement all while the operator still has his finger on the Start button.

An example where you might use the OSF would be on a conveyor where you are counting the number of product passing a given point. In this scenario, you could install a photoeye that comes on when the product enters its field of vision and then off as the product exits. Using the OSF would allow you to easily increment your counter by one (and only one) as the product exits without a lot of additonal code.

I am sure there are other (and probably better) examples, but I thought I might give it a go anyway.

STeve

Steve
 
I'm not a big OSF guy myself since I came up on the classic AB PLC5's amd they didn't have it. The biggest benefit I can see from an OSF is that it is a true falling edge instruction. You must go from ON to OFF in order to trigger the output. In chavak's post (#5) the constructed OSF equivalent may unintentionally trigger on the first processor scan if the conditions are right. You would have to add protection against that case unless a true OSF is used.

Keith
 
To OSR or NOT OSR, is that a question?

Ron, I used OSRs even before there were OSR instructions. You can program one using regular logic instructions such as XIC,XIO and OTL and OTU. However, on to your question as to why in simple terms. I have used OSRs primarily when I am working with instructions that manipulate data. These would be ADD, SUB, MOV and the like. A processor will execute an ADD instruction as long as the rung is true on each scan and that is not a problem unless you are totalizing in a register. AS in ADD N7:5 to N7:6 and put the result back in N7:6. Also, if you are moving bits around, as in say from a register to an output word, you may not want to grab that output word and hold it by virtue of having a rung be true and the move just essentially holding the output word captive. If you put in an OSR, you will get the move completed and release the output word for manipulation by other rungs and logic. All these examples are not relevant unless you need them for your particular application and you may never have need of them.
 
Steve Etter said:
Well, how about this:

An example where you might use the OSF would be on a conveyor where you are counting the number of product passing a given point. In this scenario, you could install a photoeye that comes on when the product enters its field of vision and then off as the product exits. Using the OSF would allow you to easily increment your counter by one (and only one) as the product exits without a lot of additonal code.

I'm not sure of AB, but Simatic counter works with dynamic imput and edge evaluation is not necessary, this works as well if counter caunting capacity is enough.

eye ....counter
--]/[-----(CTU)--
 
Last edited:
chavak I am a bit confused,
Can't the Input conditions can be reversed to get either OSR or OSF


Didn't want you slip by on this one. You have to remember that the scan runs top to bottom and left to right.

From the logic shown the first ouput will never make a change becuse the 2 nd output which has the same address will overwrite the value written by the preceding rung.



Drewcrew6
 
Last edited:
Hi Drewcrew,

I think I confused you, sorry, it was not a real life scenerio in the image I posted. Of Course it would be only one of the rung placed in the ladder.

I was trying to tell wouln't the output active upon rising edge in the first rung if it is there ----

Or if the second rung is the one in the ladder, wouldn't the output active upon a falling edge.
 
OSF

One Shot Falling - Old School

A B C
----]/[-----] [-------------( )

A B
----] [---------------------( )
 

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