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  • MVP 2023

Obviously there isn't enough information here to answer your question, but here is the approach I would take:

- Search for MB 7 to see if it exists anywhere else in the program (other than the Reset in rung 4). If not, then:

- While online (as you are above), try to turn off the bit. If it will turn off, then look at the logic in Rung 2 that turns on MB 3. If anything can turn on MB 3, even if only for 1 PLC cycle (which you would not be able to see while online), then MB 7 will be Set.

- If neither of the above solves the problem, then now that you have MB 7 off, do some testing to see if you can get it to turn on again and note the conditions present.

By the way, it is good programming practice to label every operand used in the program. Also, NEVER have a timer contact following the timer coil, as you are doing (twice) in Rung 4. It makes no logical sense and will not function properly. While not prohibited, it is usually poor practice to have ANY contacts following a coil (most PLC programming software won't allow it at all even though Unitronics does). Break the logic up into multiple rungs to avoid that. Think of a ladder rung as "conditions >>> actions". Keep all the conditions (contacts, compares, etc) to the left and actions to the right (coils, stores, etc).

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  • MVP 2023
1 hour ago, Ausman said:

also do a full Initialize and Reset of the plc

Glad you mentioned that Ausman. I was going to ask, and forgot, if the coil was always on or if he actually saw it turn on. I've been surprised on occasion at what coils are already set when I load software. It's usually a good idea to load a blank project and perform initialize & reset, prior to loading a new project in a PLC - even a brand-new PLC fresh from the factory.

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