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Hey All,

Just ran across a quirky little thing while working on a new project,
I'm wondering if anyone else has experienced this.....

I'm doing MODBUS control of an AB Powerflex VFD , and mind you this is not my first experience doing this, I've done several projects using V570's, and usually do not run into problems.

This time I'm building some controls around a V120, and I am having an issue reading the speed feedback (frequency) from the VFD.

I've compared my logic to a previous project (logic looks the same, only difference is the MI's used in the RHR FB), and even set up two PLC's on my workbench (V570 & V120), and hooked up each to the same VFD to test.

Problem I have is that when the net is true, I get the correct reading, but when the net is not true I get some other number in MI 13. I have searched MI 13 and it shows it only being used in the RHR FB and on the display.


I have slowed down my index to 1 Sec per step so I can see it in debug, and find that if I unplug my MODBUS cable while The net is true, then MI 13 stays put even after the net goes false, so it doesn't seem that the funny number is being driven by something in the program.


V570 reading speed feedback of 60Hz


V120 reading speed feedback of 30Hz when the index equals in that net


V120 reading something else when the index does not equal in that net


I'm starting to wonder if it has something to do with Enhanced Vision vs. Standard Vision?

Any thoughts?


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Hi John,

Are you doing any Vector Fills or Copies where the index starts below MI 13 and the length is causing MI 13 to be over written?  The find function will not show MI13 linked to the Vector Copy or Fill if it's is part of the MI's in being written to due to the length of the vector.



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Hi Cam,

Good call.......

Silly old me, I have a RHR to get the fault code from the VFD right below MI 13, and the vector length was set to 16.....



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Yes, good call, Cam.

For this very reason I always identify all used areas of vectors sequentially.  eg. 130CbusRcv1, 130CbusRcv2 etc.

I have always pondered (grumbled?) why the program does not generate such filling itself.



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