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gromit

RSLogix5000 warning

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We recently had a PLC5 converted to Control Logix. When verifying the project, the following warning appears: "Warning: Duplicate Destructive Bit Reference Detected". It looks like many instances of two one-shot "ONS" instructions that have duplicate addresses. It never appeared as a warning with RSLogix5, but does appear as a warning in RSLogix5000. What are the implications of having duplicate addresses as indicated? Thanks.

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On ONS instructions, using the same bit address will cause either, or both, to fail. As a general rule, you should always use unique BOOL tags for ONS. Only very special circumstances, such as using one of "alternative" subroutines, can you get away with duplications, but you still have to be careful that they get the correct starting conditions. Even in those special circumstances, it does no harm to have unique tags anyway. I would get rid of the duplications by creating new BOOL tags as needed. If you can post the project file, experts can determine if the duplications would cause the ONSs to fail.

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If they occur in two seperated sections that ABSOLUTELY cannot be running at the same time then you can probably get away with it. I still don't like it though.

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Why risk an issue. Just create some BOOLS (I use SYS_BOOL as an array). Then each ONS has it's own BOOL assigned. Neat and tidy......

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Guys, I like your thinking, and this is what I am gonna do. Thanks for your insight and response. Funny thing is how did this not create an issue with the original PLC-5 program?

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Without seeing the original code, no one can tell. Perhaps the ONS instructions were not actually needed, so the fact that they wouldn't have worked properly didn't make a difference. The Logix5000 duplicate destructive test may have just highlighted a bit of sloppy programming.

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