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debug mode - checking logic

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I'm a new user of Ladder Logic and M91 PLC's. I'm trying to debug a program I have uploaded off of an old PLC.

I understand that in U90 there is no way to force input/output states. Considering this how can you check that your logic is correct?

E.g. how can I check that a certain combination of inputs will cause a change of state or particular output?

[i don't consider a red flow of logic sufficient, especially as I can only see it for the power on state.]


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  • MVP 2014


If I have understood correctly, you have a couple of options, all of which involve some legwork, but will give you the results you need.

1. Get some wires and apply 24V to the inputs you need to activate.

2. Modify the program and put an MB contact in parallel with each the physical input contacts. Then you can manually toggle the MB values to simlulate inputs changing. Save a copy of the original program before making the changes.

If events happen too fast to see, you may also need to add math functions to increment a register each time the event occurs, so you know it happened.

I hope this helps.

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Thanks that was helpful.

Sadly my program won't download due to system error '9'. I've emailed the code to unitronics' support team so hopefully they can help me with it.

Another question though while we're here:

In the code I uploaded I found output set and reset coils connected in series. I understand this will work for Memory BIts, but can you do that for output coils? Surely they should be connected in parallel.


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  • MVP 2014

Unitronics does not require output conditions to be in parallel. This is different to "traditional" ladder logic. Of course it is fine if you want to put the output coils in parallel, but that will have no effect on the function of the program.

When you get the error9 does that only happen for a download, or when you save the file? You must save the file to a fixed harddisk on the computer. It will not play nicely with a network drive or removable drive.

What type of serial port are you using - USB dongle or direct RS232?

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