Comments on: Tag Based Controllers

By: ivirban

ivirban — Mon, 14 Feb 2011 21:11:43 +0000

I faced the same situation with tags such as bit[3].5; this is automatically asign if you convert a PLC5/SLC500 program in RSLogix5000. Probably it was a rewritted program for a SLC and they don’t bother to change tag names.
Actually I tested converting RSLogix500 to 5000 and there are few things to modify, I think it’s a document on AB site which tells you what you should take into account.

By: @Scott

@Scott — Mon, 16 Aug 2010 19:35:36 +0000

thx for your help!

By: Scott Whitlock

Scott Whitlock — Sun, 15 Aug 2010 14:39:03 +0000

@Peter,

I don’t see a need to make it that complicated. A Tag is interchangeable with the term “variable” in a traditional programming language. It’s any mutable value. The programmer defines what it represents, which could be a process variable.

I agree that process variable = domain value.

By: Peter

Peter — Sat, 14 Aug 2010 08:21:37 +0000

Yes, it does make sense! So you can say a process variable is a domain value and the tag the location where its stored? The plc’s provide tags which are read by the control system which could be different plc’s or supervisory software. The latter could provide the values in the tags as process variables?
Does this make sense to you
Have you ever heard of “super tags”?

By: Scott Whitlock

Scott Whitlock — Fri, 13 Aug 2010 16:19:33 +0000

@Peter,

To me, a process variable is a physical value, like speed, temperature, etc., but a tag is a “handle” for a memory storage location in a PLC or PAC. So you use tags to store process variables, but you also use tags to refer to internal state or even partial computation results.

Does that make any sense?

By: Peter

Peter — Fri, 13 Aug 2010 11:49:13 +0000

Hey, I know this article ist quite old and automation is not my custom domain.
I have to deal with a large control system and I am wondering what exactly is the difference / correlation between a TAG and a PROCESS VARIABLE?
thx