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5/27/2010 2:56 PM | |
Joined: 9/27/2006 Last visit: 9/20/2024 Posts: 12282 Rating: (2684)
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Hello Remo; That statement is basically correct, although there is one exception: A DB compiled with the attribute UNLINKED will be downloaded in the LOAD memory (MCC card) only, and will be flagged as non-relevant to the execution of thePLC program.. If you decide to go that route, here is some basic info on the subject: UNLINKED attribute for DBs: Identifying Data Blocks as "Not Relevant for Execution" Data blocks that were programmed in a source file as part of an STL program can be identified as ”Not Relevant for Execution" (keyword UNLINKED). This means that when they are downloaded to the CPU, the DBs are stored only in the load memory. The content of such blocks can, if necessary, be copied to the work memory using SFC20 BLKMOV.This technique saves space in the work memory. The expandable load memory is then used as a buffer (for example, for formulas for a mixture: only the formula for the next batch is loaded in the work memory). SFC84/83 explained: Reading and writing to DBs in Load memory (compiled with the UNLINKED attribute): http://support.automation.siemens.com/WW/view/en/26494208 See the chapter on Creating Blocks in the following manual (Programming with Step 7) to see how STL sources can be compiled with attributes: http://support.automation.siemens.com/WW/view/en/18652056 Notice the caveats: these UNLINKED DBs can be used only as buffers, the data must be read into WORK memory when needed by the program, and written back when the data is no longer requires. So they work great for recipies, or look-up tables, but not for standard pruposes. There are other ways of optimising your code that could reduce Work memory requirements (programming in STL, using multi-instances DBs,...). Hope this helps, Daniel Chartier |
Last edited by: dchartier at: 5/27/2010 2:57 PM |
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