Question about program controls

Is there a similar instruction in program controls for micrologix and SLC A/B that would be What AFI is in PLC 5? Just wondering, because I don't really see anything like that in Micrologix or SCC 5/05. Thanks

No, you will need to roll your own. See link for one example.

http://www.plctalk.net/qanda/showpost.php?p=432430&postcount=10

No, not exactly. The closest thing to the "Always False Instruction" in RSLogix500 would be a normally open contact from an unused memory bit. In other wors, a bit that never gets turned ON would do the same thing as a bit that is always false. Many programmers use such a test bit, such as B3:0/0, or B3:0/256 to temporarily inhibit rungs during testing.

Thank you!!

Lancie1 said:

No, not exactly. The closest thing to the "Always False Instruction" in RSLogix500 would be a normally open contact from an unused memory bit. In other wors, a bit that never gets turned ON would do the same thing as a bit that is always false. Many programmers use such a test bit, such as B3:0/0, or B3:0/256 to temporarily inhibit rungs during testing.

Click to expand...

And many programmers include a rung that unconditionally turns this bit OFF as the first rung of the program, so that if anyone sets it ON,

e.g.

        B3/0 

|-------(U)--|

i try to set 2 bits

# i unlatch be/0 and label it as always off.
#2 i latch in bit b3/1 and label it as always on.

when you debug your program code and forget what you did,
look for these bits.

all to often i find a shorted branch around logic i a program that has a problem.
is this branch done for a reason due to a sensor, or because the prrogram was modified due to a production change?
i also find bits that are not used that are turned off / on and no one knows why.

a little venitng i guess, but documenting your program is important. we also keep notebooks for such changes and hopefully they are filled out.

regards,
james

Hi

Just on ten always off bit .
I know I am been very odd
But let's say some one toggled the always off bit after the scan has scanned the first rung if thats where the otu but is is it not on then for the rest of the scan and then it does not work like a afi. Also maybe some where in code some one turns it on via a copy, mask move, indirect addressing etc it could cause problems


Donnchadh

If you need a temporary afi you could use an xic and an xio with the same address in the rung preceeding your other code.

b3/0 b3/0
|--] [-------]/[---------------|

Daniel.Berry said:

If you need a temporary afi you could use an xic and an xio with the same address in the rung preceeding your other code.

b3/0 b3/0
|--] [-------]/[---------------|

Click to expand...

I do this all the time using first scan bit.

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