Quick issue with RSLogix 500

[Shawn_Mac]

I've run across this situation twice now and not sure why it is happening, but I'm sure I am not the only one to see this before and maybe someone can help me avoid it in the future.

I have a drive setup for 5 different preset speeds & ramps using a combination of 3 preset inputs from a Micrologix 1000. Program controlling is a basic grafcet step by step program with a few comparison and timing bits controlling the outputs for when the different speeds are required.

What is happening is that the rung is going true on the first run sequence on this motor but the output isn't, I can force the output, but I can't even toggle the bit. It's not latch unlatch the rung goes true and the program can't move on until this step is complete so it sits in the true with output remaining false. I thought maybe it was being given run condition in two spots at once but it isn't and I'm a little lost as to why the output won't go "high" when the rung is true in this case.


Any thoughts are appreciated.

 

[brucechase]

The best bet is to post the code. There are so many good people here that you will get a great solution in a matter of a couple of days.

If the output is not used more than once, are you scanning that ladder, if it is not in the main subroutine?

 

[Ken Moore]

Based on the OP's comments, suspect he has the same outputs on two or more rungs.
While this can be made to work with conditional subroutines, it is not considered best practice.

As brucechase mentioned, posting your code will get you the most comprehensive responses.

 

[Shawn_Mac]

Ken you were right with that assumption and I didn't realize it was a problem (inexperienced yet), After I posted this I went back to testing the program and tested the machine again with all the presets on there own rung and it ran fine.

I had previously had it set up so the logic on each rung wrote to the combination of presets required to run at the selected speed so in one case there was one of the outputs on 3 rungs. Another lesson learned.

Thanks for the replies

Cheers

 

[Roy Matson]

I can't figure out why AB Ladder lets you use the same output twice. Modicon would stop you doing that, or at least it used to.
Roy