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Why is a contact energized but there is no power flow through it?

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Hello everyone,
I am new to Unitronics and I am facing a probem with a part of my ladder program.

I have attached two images illustrating the problem.



Basically, if we want O7 to be Reset, I7 and/or I17 need to be OFF and I16 need to be ON.

The problem I am facing is that if I17 is OFF and I16 is ON power flows through the contacts and resets O7 but if I7 is OFF instead of I17, power does not flow.

The same thing happens with the following lines i.e. power will flow through I17,I16,I19 to energize O3 but not through  I7,I6,I19.

Power will flow through  I17,I16,I19,I18  to energize O2 but not through  I7,I6,I9,I18.


I have reproduced the same piece of ladder in an empty file and alone it works fine.


I tried to break it to more rungs and it behaves in the same way.



I don't understand why this is happening because electrically this should work.


Any help will be much apprecited.

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In the images I did not see output 7 being used. Is it in a different part of the code?


One issue that I do see is that the same coils (Output #0-3) are being called in multiple nets. This creates the issue where the logic will process each net, but the state of the output will only reflect the last shown net. For example if there are conditions to reset Output #0 in nets 60 and 65, then only the logic in net 65 will matter. Even if the logic in net 60 tried to reset output #0, if the logic in net 65 does not trigger the output to reset than it will not be reset.


This is because the PLC processes the scan as follows.


1. Read physical inputs

2. Process ladder code

3. Update outputs


Since the outputs are only updated after the entire ladder is run, this will cause the last instance of an operand to be the determining condition,

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  • MVP 2023

I have reproduced the same piece of ladder in an empty file and alone it works fine.


This says you have duplicate coils somewhere on your project. As Alexander says above, the last instance of a coil overrides previous instances. Right-click on the label box of the coil that is not behaving as expected and select "Find...". Check each instance of the coil that shows up in the search results box at the bottom of the screen.

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