String clearing

[Neuling 0]

Hello,

Is there a command to clear the content of a string in rslogix 500?

I'm scanning a barcode string to ST9:0. After that I need to remove the content of that string for the next scan. So far I have copied an empty string into ST9:0 to make it work.

Shouldn't I be able to use the CLR command? When using the CLR command with destination ST9:0 I'm getting an error address must be specified to world level!

 

Thanks for any help

Edited 19 Nov 2017 by Neuling

[b_carlton 61]

A string is a defined type composed of a length then an array of SINTs. I believe the CLR only works on atomic types (sint, int, dint etc). Copying an empty tag is my go-to method for more complex types. You may get some effect by just setting the length to zero but when I tried that I would occasionally see problems with new characters followed by the previous characters. I feel it's best to just clear the whole thing by copying an empty string as you are doing.

Edit - actually I think CLR can be used successfully with timers and counters so my blanket statement is probably false. It would be good to have a CLR with respect to strings.

Edited 19 Nov 2017 by b_carlton

[Neuling 0]

Thanks for your answer

[PlcAutomation 0]

Try "FLL". Source 0, Dest String.DATA[0], length, string.LEN

[pturmel 157]

You can clear just the .LEN element.

[BobLfoot 301]

@pturmel unless Rockwell has changed the behavior, setting the .LEN to zero hides the string from displaying, but the .DATA[x] registers retain their ascii values.  What @PlcAutomation recommends combined with a Mov 0 String.Len totally clears things.

[pturmel 157]

Yes, just setting length to zero doesn't rewrite the data bytes, but all logix string instructions honor .LEN and will not use those ignored bytes.  Well-behaved OPC servers will also ignore those bytes.

[PLCMentor.com 60]

It seems that copying an empty string solves the problem and is simple.  Is there a problem with that method that would require any of the other methods discussed here?

[pturmel 157]

You have to ensure the empty source string stays empty.  Marking it as read-only or a constant is probably sufficient.

[pturmel 157]

21 hours ago, BobLfoot said:

What @PlcAutomation recommends combined with a Mov 0 String.Len totally clears things.

BTW, this can be simplified to just a single FLL targeting the .LEN element with a constant length of 22 42.

{Edit: Whoops!  SLC, not Logix.}

Edited 23 Jul 2021 by pturmel
Wrong processor type.