FFL confused

irondesk40 · Feb 2, 2009

Confused trying to use the FFL and FIFO
In the attached example, I thought everytime that that B3:0/12 would go true that the value in N7:60 would moved into a stack starting at N14:0
I placed a value of 5 in N7:60 then triggered B3:0/12 and monitored the N14 and never saw the 5 placed in N14.

Could someone tell me what I may be doing wrong.

Thanks

Christoff84 · Feb 2, 2009

From the look of your example its quite possible you are loading the FIFO correctly and then immediatly unloading the FIFO stack.

When you try this, does N7:10 have a value of 5?

OkiePC · Feb 2, 2009

Christoff is most likely correct. When using a FFU, FFL pair, I always put the FFU immediately preceding the FFL in the PLC scan (usually as a branch one the same rung so someone doesn't inadvertenly insert logic between them).

By doing that, your FFL can load data, and it will be present for one full PLC scan for use by other parts of the program.

Without knowing the intended usage of your fifo stack, I can't really go much further than that, but there are some other gotchas that you may run into, depending on what you're doing with the data.

Paul

kamenges · Feb 2, 2009

I also agree with Christoff84. As soon as you load a value into the FIFO stack the stack is no longer empty. So if your saved version is accurate R6:1.EM will become false on the load and trigger an immediate unload.

Keith

irondesk40 · Feb 2, 2009

Thanks alot to everyone.
I am actually looking at do a chemical tank system and looking the FFL in order to fill the tanks and know which tank became empty first. Had never used the instruction, and I was thinking everytime the condition went true for the instruction it would execute on the leading edge. I changed it to use a pulse to trigger the FFL and FIFO (after going to lunch and reading the instructions agin, I finally understood what I was doing wrong), just one of those days.

I had tried to call tech connect to see if they could tell me what I might be doing stupid, but after being on hold for over 30 minutes, just gave up and posted on her.

This site is great

timbo_uk · Mar 12, 2009

irondesk40 said:
Thanks alot to everyone.
I am actually looking at do a chemical tank system and looking the FFL in order to fill the tanks and know which tank became empty first. Had never used the instruction, and I was thinking everytime the condition went true for the instruction it would execute on the leading edge. I changed it to use a pulse to trigger the FFL and FIFO (after going to lunch and reading the instructions agin, I finally understood what I was doing wrong), just one of those days.

I had tried to call tech connect to see if they could tell me what I might be doing stupid, but after being on hold for over 30 minutes, just gave up and posted on her.

This site is great

Hi there, I am also learning the FIFO usage, have you got a print of your modified ladder as I think I have done the same as you did too! (http://www.plctalk.net/qanda/showthread.php?t=45715)

Cheers
Tim

FFL confused

irondesk40

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Christoff84

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OkiePC

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kamenges

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irondesk40

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timbo_uk

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