Schneider: memory words and constant words

[Varri]

Hi!

I know there is a lot of skilled people with Schneider somachine in this forum, so i have to ask something.

How and why do you use constant words instead of memory words?

I was not able to find it specified in the manual, is constant words stored in the flash memory?

Lets say that i have a machine with 30 paramterers adjustable by the operators, as retained memory tags. Retained memory words?

And i have 60 parameters, formula constants and such things that never changes. Constant words?

 

[Saffa]

Pretty much.

Changing a constant requires a rebuild (in Control Expert, not sure about Machine Expert) which is sometimes a pain during commissioning but if you're certain it's set and forget then it's a safe way to make sure someone doesn't accidentally modify something they shouldn't.

I -think- that Constants can't be assigned %MW addresses so if you need to display them on an HMI then not ideal.

 

[Varri]

Thank you for your reply.

Whats the point of constant words then? Surr the capsity for ordinary memory word is plenty.

I like write parameters for a function first in the block its used for. Write em every scan or first scan.

 

[parky]

My interpretation of a constant is rather than it being a memory it is a fixed value i.e. part of the code I have no experience of Schneider but in many platforms you have the following
Variable // A memory area where the data can be changed
Variable Constant // A memory area where it contains a constant i.e. a fixed value
A constant // Hard coded value in the code
so examples
ADD (My_Label, Another_Label, Result_Label) Adds one var to another
ADD (My_Label, Constant_Label, Result_Label) does the same thing only one is a constant stored in a variable)
ADD (My_Label, 23.45, Result_Label) a variable is added to a fixed constant.

 

[Lare]

Thank you for your reply.

Whats the point of constant words then? Surr the capsity for ordinary memory word is plenty.

I like write parameters for a function first in the block its used for. Write em every scan or first scan.

I haven't used constant variables never on Unity or CE(started with Unity 2.3 version over ten years ago).
Instead I used fixed constants numbers or variables with start values or initing variables on code.
With that way you can modify with build changes, no need to build all, even change remotely if needed.

Constants are probably for compatibility for P57 PL7 series or even older platform. At that time there was less memory words available.

(Was concept PLC programs able to convert to PL7 or Unity platform with converters?)