Purpose of alias tags in RSlogix5000

[CapinWinky]

It should be pointed out that programmers coming from other platforms might think that an Alias is a pointer. It is not a pointer, it is just a define that is replaced by the preprocessor with the actual variable name. Basically Rockwell has chosen to not let you rename the process variables they automatically assign to IO and so have created a mechanism to call them something else while in the editor.

They did some extra steps at some point to store this alias information in a table on the PLC so you can see the alias names when you connect online, even if you don't have the original project files. It's a pretty silly way to do it and you can tell this has completely gotten away from them. They can't figure out a way to make it work with structures (UDTs) and there are other limitations that they haven't found a way around. At some point, they will probably just make Aliases pointers because it would solve all their shortcomings with the system, but for now they are not pointers.

 

[kamenges]

Originally posted by RET:

My personal opinion is to never use aliases.

At least a little supporting argument for that opinion would be helpful. I'm not saying you're wrong but it is hard to say you are right without knowing why you feel that way.

Keith

 

[pjst0rer]

Thanks CapinWinky.
Actually I don't care too much about aliases but I take your point. It would be easy enough to just map the Io to readable tag names in the plc. My problem is I want to avoid breaking the links to the hmi pvp when I change the tag name so I would like to see my tag descriptions in the FT view tag browser. Is this possible or are they just teasing me by showing a description field in the browser?

 

[pjst0rer]

Actually Ken Roach answered this question back in 2011, but I thought thinks might have changed since then...

 

[JaxGTO]

I just wish AB would do what they said they were going to do in version 9. Alias a sub element of a UDT. So you don't need a bunch of rungs or an AOI to load aux input into a motor starter tag etc.

 

[RET]

At least a little supporting argument for that opinion would be helpful. I'm not saying you're wrong but it is hard to say you are right without knowing why you feel that way.

Keith

Aliases are okay, only if you're doing a one-off program for a very specific job that will unlikely see another application. However for programs that will be used as a go-by or re-purposed for another, similar but not identical application, they are horrible to work with when back tracking and modifying a program.

 

[baldone]

Aliases are okay, only if you're doing a one-off program for a very specific job that will unlikely see another application. However for programs that will be used as a go-by or re-purposed for another, similar but not identical application, they are horrible to work with when back tracking and modifying a program.

+1....the fact that you can not do online changes to the alias makes them pretty much useless for me.