RS5000 & FSC Instruction

Good day all,

Got a quick question hopefully someone can answer. OK, I am relatively new to the ControlLogix Platform. Mainly been dealing with SLC500's & PLC5's. I have been assigned to a project, where I am reviewing plc code for the equipment coming in for the project. A section of the code uses multiple FSC instructions. No problem. I understand the file search/compare instruction and what they are intending to do with it. What I am having trouble with is that the Control Word being used for each instance of the instruction is the same (e.g stack_search). Now, if I remember correctly, in the older Rockwell platforms, this is not recommended. (e.g Should not use R6:0 twice.) Is this true for the ControlLogix Platform? Or is this perfectly acceptable??

Thank-You,

Andrew Evenson

If the FSC is set up to do the whole search in a single scan (ALL mode), and the control word is referenced before the next FSC occurs, then it may be acceptable to do it this way.

Paul

Greetings Andrew ...

does the program make any use of the .FD (Found) bit? ... or of the .POS (Position) member? ... if the answer is "yes" then you could have a problem - especially if the FSC could conceivably find more than one TRUE match ...

I think this is what my distinguished colleague OkiePC was getting at too ...

basic idea: suppose that the first FSC finds a match, and that this particular match needs to be counted and its location/position recorded ... and then that same FSC needs to pick back up where it was and continue the search ... in that case, the next FSC could possibly "reset" the SINGLE control structure before the first search could be resumed ...

disclaimer: this is all based on how I interpret what you've described ... if possible, you might consider posting the part of the program that's involved in this operation ...

Possibility ...

Are the multiple FSC's in seperate programs within the task? If so, are the control words program scoped, or controller scoped? If they are program scoped, then they would be considered seperate, but unavailable to other programs outside of the one that they are scoped for.

good thought, Oakley ... but when Andrew said "a section of the code" I assumed (gosh I hate that word) that the FSC's are more or less right there in a row - not in separate programs ...

yet another reason why seeing the program would help with the analysis ...

Ron Beaufort said:

yet another reason why seeing the program would help with the analysis ...

Click to expand...

It sure would help with analyzing questions to provide answers. I hate to make assumptions too.

Hopefully Andrew will chime back in and give more information.

I am reviewing plc code for the equipment coming in for the project

Click to expand...

Which has obviously been tested/simulated etc.

I could adopt the stance "If it ain't broke - don't fix it".

It is quite in order to "re-use" the same control structure for a chain of file-based instructions. This is particularly true where a "scalable" data architecture is required, there may be a single MOV to the .LEN of the control structure to set the limits of all the FSC or FAL operations. This could be true in your case - the code is "coming in", and could well be generic for differently-sized plant.

Most times a RES instruction prior to the next usage is performed, so the next FSC or FAL performs as if it were in isolation.

Occasionally some clever programming uses the control structure as it was left by the previous instruction without a RES, and, providing it does what is required, it is also valid.

If it worries you greatly, you may be able to ask the code developers to explain their logic to you, especially if your company will have to maintain the code after commissioning.

HTH

Thank you all for replying!

Goodday!

Thank you all for your replies! It has certainly made me think a bit about the situation.

To answer Ron & Oakley: Yes, the FSC instructions are in the same ladder task. Unfortunatley, I cannot post the code, as I don't know/want to break any legal agreements between vendor and us.
(Basically, I don't know if I can legally post the code..)

In looking at it further, I think daba might have hit the nail on the head. The FSC instructions are called via "LBL/JMP" instructions. Once the search is complete, the ".EN", ".DN". ".IN" are unlatched, and the ".POS" is set to zero. This now is leading me to believe that there is no reason why using the same control register can't be possible. In my case, if the instruction is reset before the next one is excecuted, I guess there should not be a problem.

As well, daba: You are correct, I'm not second guessing that this won't work. I just thought you were not suppose too do it this way.

Again, I thank everyone for there help and input. This is certainly a great forum!!!

Andrew Evenson