AFI help

Having trouble getting this afi instruction to go true.
How do you program this thing to go true?

AFI
-[/]-


p.s. your windows logon needs sobriety test not password!

ganutenator said:

Having trouble getting this afi instruction to go true.
How do you program this thing to go true?

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"Always False Instruction"

since it's normally FALSE - you have to enter it backwards to make it TRUE ...

try putting it in as ---[IFA]--- and see if that works ...

Ron Beaufort said:

since it's normally FALSE - you have to enter it backwards to make it TRUE ...

try putting it in as ---[IFA]--- and see if that works ...

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Excellent!


Ron, could you please give a description of putting a TRUE IFA memory in a Bit Box?

You laugh, but you probably haven't put it outside an mcr circuit in a subroutine w/ an unlatch and only tried to call it on prescan neither. so judge not.

You can't be too safe.

not sure whether this is serious or not - but just in case it is, maybe this will (hopefully) clarify:

an "Unlatch" (specifically, an OTU instruction) doesn't/won't/can't "do" anything on Prescan - whether the rung conditions leading up to the OTU are TRUE or whether they're FALSE doesn't make any difference ...

specifically, during Prescan the OTU instruction will NOT write to its assigned bit/box ... no ONE ... no ZERO ... no NUTHIN ...
.

Ron Beaufort said:

not sure whether this is serious or not - but just in case it is, maybe this will (hopefully) clarify:

an "Unlatch" (specifically, an OTU instruction) doesn't/won't/can't "do" anything on Prescan - whether the rung conditions leading up to the OTU are TRUE or whether they're FALSE doesn't make any difference ...

specifically, during Prescan the OTU instruction will NOT write to its assigned bit/box ... no ONE ... no ZERO ... no NUTHIN ....

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...because the prescan doesn't "solve" the logic, it just resets non-retentive "bit-boxes" (those that are attached to OTE instructions).

...one day Ron will refer to his bit/boxes as "memory locations". That might be a sign the world is about to end, lol

I was also wondering if the OP's post was serious, but then remembered one of my class asking if there was an "Always True" instruction..... I showed him an unconditional rung and said there were billions of them in series before the output instruction. It took a few seconds before he got the point !

some other brands (I seem to remember an old Texas Instrument platform) actually did have an Always True instruction ...

you needed that function in case you wanted to "strap out" or "bypass" a certain condition ...

Allen-Bradley lets you put in an UNCONDITIONAL branch to do the bypass - but some brands won't let you do that ... I suppose the software believes that it wouldn't make any sense to put in an "OR" branch with no "condition" in it - so the Always True instruction was provided as a workaround ...

one day Ron will refer to his bit/boxes as "memory locations". That might be a sign the world is about to end, lol

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in that case, then the world will never end ...

actually from the way I look at it, there are a LOT of different types of memory locations ... INTs, DINTs, REALs, etc. ... whenever I refer to a "bit/box" I'm invariably drilling down to the basic unit level - a "box" which can only hold either a status of ONE - or of ZERO ...

so yes, a "bit/box" is indeed a "memory location" - but the terminology that I use seems to help people get a better grip on the ideas involved ...

peace ...

Ron Beaufort said:

some other brands (I seem to remember an old Texas Instrument platform) actually did have an Always True instruction ...

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The Automation Direct DL series has a collection of SPecial relays. Among these are Always ON and Always OFF.

lol, of course I was joking from the onset.

But what a lovely discussion this has turned into.

p.s. It's been years since I touched an AB product, but didn't they set the EN bit on stuff during the prescan?

Doug-P said:

The Automation Direct DL series has a collection of SPecial relays. Among these are Always ON and Always OFF.

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your Avatar rocks!

ganutenator said:

your Avatar rocks!

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Avatar still updates while Doug-P (DOUG-PLC) is offline. Hmmm. How does that work?

but didn't they set the EN bit on stuff during the prescan?

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actually Prescan will write a ZERO to the EN (Enable) bit/boxes of most instructions ...

it also will write a ONE to the CTU (Count Up) bit/boxes of most Counter instructions ...

it also will write a ONE to the bit/boxes assigned to most ONS (One Shot) instructions ... (at least in the RSLogix5 and RSLogix5000 systems - but not in the RSLogix500 systems) ...

and just for added weirdness, there's also a slight difference in how Prescan handles "unused/unscanned" routines in the RSLogix5000 systems - and how those are handled in the RSLogix5 and RSLogix500 systems ...

and so on - and so on ...

Avatar still updates while Doug-P (DOUG-PLC) is offline. Hmmm. How does that work?

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Doug winds it up tight before he logs off ... if ganutenator will sit quietly and stare at it long enough, he'll eventually see it slow down - and then finally stop ...