Anyway, back onto the subject and I’ll hopefully be able to make it clearer what I mean.
Put simple, ControlLogix user data memory is just one big file (aka Tag database) in which you create your “tags” (well, one big global data file, every routine can also use local memory).
It has all the IEC compliant features such as Arrays (3 dimensional is max though), STRUCT’s, UDT’s etc.If you need a new (global) Tag (e.g. a bit, INT, REAL or whatever) you simply create a new one (as you program), give it a symbolic name and off you go.
It is indeed very similar to what you can do with the Local (TEMP) memory in an FC or FB.
Sounds too easy and great doesn’t it?
Well, here are things that rub me the wrong way about it:
In Rockwell’s own words “In the RSLogix 5000 programming environment, data is set up in a tag database. Memory addresses are hidden from the programmer, which makes things easier for the programmer.”
Call me old fashioned, but I don’t get how pure symbolic addressing makes things easier compared to being able to do either symbolic programming or address based programming (which S7 allows and you can freely switch between the two).
Secondly, a half decent size program can easily have a few thousands tags in it.
With Controllogix, the only distinguishing factor between each tag is their name (remember, no addresses anymore), or in other words, that Tag database easily becomes a gigantic mess as far as I’m concerned.
S7 DB functionality on the other hand allows you neatly structure your user data area in two ways (inside the DB’s AND by being able to have more than one DB).
Add to that the availability of M bits and Local (TEMP) data and even the program with the biggest data memory requirements can still be structured in a easy to identify format (e.g. if you look for Alarms you would give the “Alarm DB” the name “Alarms”, I think by now you get what I’m trying to explain).
One last things, this thread is of course seriously Siemens biased (and so it should be, we are not really here to promote Rockwell ,are we?) and there are properly features in ControlLogix 5000 that Step7 could benefit from.
Hi Fritz,
I think some part from the above is coming inside my mind and some part is going over my mind( because of lack of experience in having practically done the above which you mentioned). Any how I appreciate I got this information and sooner I will try to implement the way you have mentioned.
However I appreciate you efforts for trying to explain the things as I requested.
Last but not the least I would like to comment on the below one:
No worries lalit, the majority A/B fans that I’ve met in my life don’t get it either when I try to give them valid reasons for abandoning their “toys” and start programming real PLC’s (such is the power of the dark side of the force or my inability to explain things properly).
I have seen several people stating the same things for either AB or Siemens.
I would say it, people sometimes are reluctant to change though I might be wrong as I am a human being.
But let's take the example of any one if a person is working on AB PLC. If he is experienced for 20 years in AB, how positively he tries to learn any other PLC but it will take some time for him to learn Siemens and he will always try to find AB in Siemens.
And vice versa is true as well.
I know there are certain things for which you can Call A/B as "toys" but the same thing people might say about "Siemens".
Any how as well said> we are here to talk about Siemens and Certainly I have started finding new features in Siemens which other PLC's might not have.
Thanks, I hope to ask few more questions/clarification here on this forum.
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