ControlLogix AOI Constants

[devo.xx]

Working with AOIs, I want to create a Tag that has a constant value. I see that "Constant" is greyed out for all Locals.
Is the only way to make it a constant is to write a rung with a simple MOV? Or is there some other setting I need to set to create constants?

Logix Designer 21.03
ControlLogix 1756-L71 21.11

 

[bernie_carlton]

Create a Local tag and assign a 'Default' value.

 

[devo.xx]

Create a Local tag and assign a 'Default' value.

But that's only a default.
If I accidentally add something else to my AOI, say another MOV, or ADD (etc.) then I can overwrite it.
With a constant, at compile time, the program will report that I'm trying to change it, sort of a safety.

 

[bernie_carlton]

OK, I guess I've never had the need arise for special protection of that nature.

 

[devo.xx]

OK, I guess I've never had the need arise for special protection of that nature.

It's just a sanity check really, incase I "fat finger" the tag in the drop-down dialog.
Just seems really strange that you can't mark them as Constant, even though the box exists....

 

[bernie_carlton]

Declare it in the Controller Tags then use it as an input parameter.

 

[Usireland]

Declare it in the Controller Tags then use it as an input parameter.

Better yet, use it as an InOut parameter and you wont use more memory for it.

 

[bernie_carlton]

As just an Input parameter it removes one more possibility of the AOI overwriting it.

 

[radfahrer]

Make Input and check the Req. box then enter the value in the instance

If you make it an input parameter and check the Req. box in the AOI, then in the instance of it in your code, you will get the ? looking for a parameter reference. instead of putting a tag reference in there, just put the value you want for the constant. This is how I do ranges for my AI AOI's and it works nicely.

 

[devo.xx]

If you make it an input parameter and check the Req. box in the AOI, then in the instance of it in your code, you will get the ? looking for a parameter reference. instead of putting a tag reference in there, just put the value you want for the constant. This is how I do ranges for my AI AOI's and it works nicely.

Yep, this is what I was looking for.
Still seems kind of silly that I can't just set it in the Parameters Tab...

 

[TConnolly]

Since nothing is truly un-alterable by the programmer (even constants and hard coded values can be changed) just use a local tag and default value. If you think you might forget and change it then name it CONSTANT or DONOTCHANGE to draw attention to the need to leave it alone.

 

[radfahrer]

I usually name mine const_xxx and use a MOV statement in the first rung of the routine it's used in to set them as hardcodes. Has worked well for me.

 

[daba]

Just how far do you go to prevent a "fat-finger" occurrence ?

Hard-code it in the AOI.. MOV xxx Constant, the AOI code can't be changed online, so it will be protected.