Excellent features of Siemens which AB Control Logix Does not have

Phantom75

Hey Lalit & Ritu!


IMO, one area Siemens can improve is documentation. We have come a long way if I compare with Siemens documentationof early 80's but stillsome way to go...

Regards

Dear all

it's been over a decadesince I last touched PLC5 and a few years since I was last "forced" to be involved in aControlLogix project(I called this project a "downgrade" from S5 to A/B).
I believemy key "learning’s" from this experience are still valid for the latest generation ControlLogix/RSLogix butcan't guarantee this though.
It would be great if someone with the latestA/B gear can verify this since I promised myself that this was definitely the last time I touch A/B gear.

Anyway,based onmy experience there are two key things missing in lalit's list:

1.) Online Block monitoring

Siemens PLC:
In a Siemens PLC (dating back to S5 days) online Block monitoring displays the status of the monitored logic as it is when executed by the PLC.
Take the following examples(I need to make this point strongly, as this isTHE single most important feature in a Siemens PLC as far as I'm concerned):
AMemory Bit that is used in a couple ofNetworksand for instance SET to "1" inNW1 but reset to "0" in NW2, will display the status correctly for both positions!!!!
A single TEMP variable that may be used a number of times in a Networkfor intermittent result storage of forexample a mathematical calculations will nonetheless show the value that it holds correctly for each usage in all locations!!!!
I call this simply the "What you see is what you get" principle (and we all seem to take it for granted).

To do so, the CPU uses so called trigger points for online block status monitoring andneeds to work a bit "harder". Thisis one of the reasons thatS7-300 thus far allow you to monitor only one Block at any given time (to be changed soon, with the release of thelatest 300's and firmware V3.0 you'll be able to monitor 2 Blocks at the same time, just like a 400 can).

A/B PLC:
Supply of the actual status of Bits, Bytes, INT's etc.of the logic duringonline Block monitoring is donevia simpleread out of the internal data memory whenever the comms allow to do so (i.e. similartoa Variable table monitoringin an S7, except you can decide if it is updated at the start or end of the scan).
I call this the "What you see is NOT necessarily what you get" principleand I find it utterly frustrating and confusing (as a matter of fact, you will see live status of the logic even if the subroutine is currentlyNOT called).

2.) Data Blocks
This concept simply doesn't exist in a ControlLogix PLC.
While you can create structures and the likes in the "flat" memory of a ControlLogix, it just doesn't cut it for me and isn't the same as being able to do what you can do with DB's.

Anyway, that's my two cents worth.

fritz

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 tagis 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.