AB's One Shot Falling (OSF) Instruction

Andrew Evenson · Jun 11, 2003

Hi all,

I'm trying to dectect the true to false transistion of a bit.
I'm using a PLC5-40/E processor. I've decided to use the OSF instruction, but for the like of me just dont understand the instruction book's explination.

Here is what I think the code should look like...



                  ------OSF--------------------

    B3:0/6       |Storage Bit : B3:10/0        |               

-----||----------|Output Bit  : 0              |

                 |Output Word : B3:11          |

                 -------------------------------

Now in the event of B3:0/6 going from True to false, should I see B3:11/0 go high for 1 Scan?? or would ot be addressed as B3:11.OB for "Output Bit"??

Am I missing the boat here?? If anyone can give me some insite on how this instruction excecutes, I'd really appriciate it..

Andrew Evenson

Andrew Evenson · Jun 11, 2003

Sorry..

Sorry, didnt think the attatchment would come out so small....

Eric Nelson · Jun 11, 2003

Your expected result from your code sounds correct to me, but I have never used a PLC5, so don't trust my judgement...

Sounds like a dopey way to go about a simple OSF instruction. IOW, why they require you to separately enter the word AND bit-of-word is beyond me...

beerchug

-Eric

93lt1 · Jun 11, 2003

Andrew Evenson said:
I'm trying to dectect the true to false transistion of a bit.

Does it have to be "true to false" transition?
How about the bit's "1" to "0" state transition?

I have never seen a reason to use the OSF instruction, so I can't tell you how it works, but I would do it this way:

Kim Gold · Jun 11, 2003

Andrew:

In your example B3:11/0 will be true for 1 scan.
B3:10/0 will follow the rung status.
I must say though that we use a lot of OSR instructions in one of programs and I find them quite confusing. You always need to stop and think, is this the bit that follows the rung status or is it the one shot. It will help a lot if you label them accuratly.
i.e. B3:11/0 - CHUTE UP TO GREASE POSITION ONE SHOT FALLING
B3:10/0 - CYCLE CHUTE UP TO GREASE

I tried to blow the pic up a little.
Hope it turns out.

Kim

93lt1 · Jun 11, 2003

Got me thinking....

Anyone know of a reason that you would use a falling edge one-shot for a different result than switching the preceding logic with a rising edge one-shot, like I showed above?

I don't think I have ever been programming a PLC and said, "I wish I could use a falling edge one-shot for this".

Eric Nelson · Jun 12, 2003

I agree 93lt1, and I make it a point to never use OSFs (or it's equivalent in other brands, i.e. DIFD).

Then again, I never use OUT NOTs ---(/) either...

I think it's more of a "user's choice" issue. It might make more sense in someone's head to use a OSF, so why NOT make it an available instruction?

With A-B, you have a choice of GRT, GEQ, EQU, NEQ, LES, and LEQ, but could manage just fine with only GRT, EQU, and LES... :nodi:

beerchug

-Eric

93lt1 · Jun 12, 2003

Eric Nelson said:
With A-B, you have a choice of GRT, GEQ, EQU, NEQ, LES, and LEQ, but could manage just fine with only GRT, EQU, and LES...-Eric

About as confusing as the TOF. I hate those. Every time I run across one, I have to get into 'help' and see how the timing vs. bit status works.

rogerhollingsworth · Jun 12, 2003

93lt1

Ditto

rogerhollingsworth · Jun 12, 2003

OSF

Andrew
Just curious
What is the nature of your process whereas you would need/want to use OSF? Are you using it due to its prescan operation?

Roger

93lt1 · Jun 12, 2003

Eric Nelson said:
With A-B, you have a choice of GRT, GEQ, EQU, NEQ, LES, and LEQ, but could manage just fine with only GRT, EQU, and LES...-Eric

PLC5 has a CMP (compare) instruction which could eliminate all of these, not taking into consideration instruction execution time.

AB's One Shot Falling (OSF) Instruction

Andrew Evenson

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Andrew Evenson

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Eric Nelson

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93lt1

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Kim Gold

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93lt1

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Eric Nelson

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93lt1

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rogerhollingsworth

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93lt1

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